Firefox 70 — a bountiful release for all

Firefox 70 is released today, and includes great new features such as secure password generation with Lockwise and the new Firefox Privacy Protection Report; you can read the full details in the Firefox 70 Release Notes.

Amazing user features and protections aside, we’ve also got plenty of cool additions for developers in this release. These include DOM mutation breakpoints and inactive CSS rule indicators in the DevTools, several new CSS text properties, two-value display syntax, and JS numeric separators. In this article, we’ll take a closer look at some of the highlights!

For all the details, check out the following:

Note that the new Mozilla Developer YouTube channel will have videos covering many of the features mentioned below. Why not subscribe, so you can get them when they come out?

HTML forms and secure passwords

To enable the generation of secure passwords (as mentioned above) we’ve updated HTML <input> elements; any <input> element of type password will have an option to generate a secure password available in the context menu, which can then be stored in Lockwise.

For example, take the following:

<input type=”password”>

In the Firefox UI, you’ll then be able to generate a secure password like so:Context menu showing password generation option

In addition, any type="password" field with autocomplete=”new-password” set on it will have an autocomplete UI to generate a new password in-context.

Note: It is advisable to use autocomplete=”new-password” on password change and registration forms as a strong signal to password managers that a field expects a new password, not an existing one.


Let’s turn our attention to the new CSS features in Firefox 70.

New options for styling underlines!

Firefox 70 introduces three new properties related to text decoration/underline:

  • text-decoration-thickness: sets the thickness of lines added via text-decoration.
  • text-underline-offset: sets the distance between a text decoration and the text it is set on. Bear in mind that this only works on underlines.
  • text-decoration-skip-ink: sets whether underlines and overlines are drawn if they cross descenders and ascenders. The default value, auto, causes them to only be drawn where they do not cross over a glyph. To allow underlines to cross glyphs, set the value to none.

So, for example, the following code:

h1 {
  text-decoration: underline red;
  text-decoration-thickness: 3px;
  text-underline-offset: 6px;

will give you this kind of effect:

a heading with a thick, red, offset underline that isn't drawn over the heading's descenders

Two-keyword display values

For years, the humble display property has taken a single value, whether we are talking about simple display choices like block, inline, or none, or newer display modes like flex or grid.

However, as Rachel explains, the boxes on your page have an outer display type, which determines how the box is laid out in relation to other boxes on the page, and an inner display type, which determines how the box’s children will behave. Browsers have done this for a while, but it has only been specified recently. The new set of two-keyword values allow you to explicitly specify the outer and inner display values.

In supporting browsers (just Firefox at the time of writing), the single keyword values we know and love will map to new two-keyword values, for example:

  • display: flex; is equivalent to display: block flex;
  • display: inline-flex; is equivalent to display: inline flex;

Rachel will explain this in more detail in an upcoming blog post. For now, watch this space!


Now let’s move on to the JavaScript.

Numeric separators

Numeric separators are now supported in JavaScript — underscores can now be used as separators in large numbers, so that they are more readable. For example:

let myNumber = 1_000_000_000_000;
console.log(myNumber); // Logs 1000000000000

let myHex = 0xA0_B0_C0
console.log(myHex); // Logs 10531008

Numeric separators are usable with any kind of numeric literal, including BigInts.

Intl improvements

We’ve improved JavaScript i18n (internationalization), starting with the implementation of the Intl.RelativeTimeFormat.formatToParts() method. This is a special version of Intl.RelativeTimeFormat.format() that returns an array of objects, each one representing a part of the value, rather than returning a string of the localized time value.

const rtf = new Intl.RelativeTimeFormat("en", { numeric: "auto" });

rtf.format(-2, "day"); // Returns "2 days ago"

rtf.formatToParts(-2, "day");

​  { type: "integer", value: "2", unit: "day" },
​  { type: "literal", value: " days ago" }

This is useful because it allows you to easily isolate the numeric value out of the string, for example.

In addition, Intl.NumberFormat.format() and Intl.NumberFormat.formatToParts() now accept BigInt values.

Performance improvements

JavaScript has got generally faster thanks to our new baseline interpreter! You can learn more by reading The Baseline Interpreter: a faster JS interpreter in Firefox 70.

Developer tools

There is a whole host of awesome new things going on with Firefox 70 developer tools. Let’s find out what they are!

Inactive CSS rules indicator in rules panel

Inactive CSS properties in the Rules view of the Page Inspector are now colored gray and have an information icon displayed next to them. The properties are technically valid, but won’t have any effect on the element. When you hover over the info icon, you’ll see a useful message about why the CSS is not being applied, including a hint about how to fix the problem and a “Learn more” link for more information.

For example, in this case our grid-auto-columns property is inactive because we are trying to apply it to an element that is not a grid container:

a warning message saying that grid-auto-columns isn't being applied because the element doesn't have display: grid applied

And in this case, our flex property is inactive because we are trying to apply it to an element that is not a flex item. (Its parent is not a flex container.):

A warning message saying that the flex property is inactive because the element's parent is not a flex container

To fix this second issue, we can go into the inspector, find the element’s parent (a <div> in this case), and apply display: flex; to it:

using the rules inspector in firefox to fix the problem with display: flex

Our fix is shown in the Changes panel, and from there can be copied and put into our code base. Sorted!

the changes panel, which can be used to copy the code changes you made, and then paste them back into your codebase

Pause on DOM Mutation in Debugger

In complex dynamic web apps it is sometimes hard to tell which script changed the page and caused the issue when you run into a problem. DOM Mutation Breakpoints (aka DOM Change Breakpoints) let you pause scripts that add, remove, or change specific elements.

Try inspecting any element on your page. When you /ctrl + click it in the HTML inspector, you’ll see a new context menu item “Break on…”, with the following sub-items:

  • Subtree modification
  • Attribute modification
  • Node removal

Once a DOM mutation breakpoint is set, you’ll see it listed under “DOM Mutation Breakpoints” in the right-hand pane of the Debugger; this is also where you’ll see breaks reported.

the degugger interface showing paused dom mutation code after a DOM mutation breakpoint was hit

For more details, see Break on DOM mutation. If you find them useful for your work, you might find Event Listener Breakpoints and XHR Breakpoints useful too!

Color contrast information in the color picker!

In the CSS Rules view, you can click foreground colors with the color picker to determine if their contrast with the background color meets accessibility guidelines.

a color picker showing color contrast information between the foreground and background colors

Accessibility inspector: keyboard checks

The Accessibility inspector‘s Check for issues dropdown now includes keyboard accessibility checks:

The firefox accessibility inspector, showing acessibility check options; contrast, keyboard, or text labels

Selecting this option causes Firefox to go through each node in the accessibility tree and highlight all that have a keyboard accessibility issue:

the accessibility inspector showing the results of keyboard checks - several messages highlighting where and what the problems are

Hovering over or clicking each one will reveal information about what the issue is, along with a “Learn more” link for more details on how to fix it.

Try it now, on a web page near you!

Web socket inspector

In Firefox DevEdition, the Network monitor now has a new “Messages” panel, which appears when you are monitoring a web socket connection (i.e. a 101 response). This can be used to inspect web socket frames sent and received through the connection.

Read Firefox’s New WebSocket Inspector to find out more. Note that this functionality was originally supposed to be in Firefox 70 general release, but we had a few more bugs to iron out, so expect it in Firefox 71! For now, you can use it in DevEditionplease share your constructive feedback!

About Chris Mills

Chris Mills is a senior tech writer at Mozilla, where he writes docs and demos about open web apps, HTML/CSS/JavaScript, A11y, WebAssembly, and more. He loves tinkering around with web technologies, and gives occasional tech talks at conferences and universities. He used to work for Opera and W3C, and enjoys playing heavy metal drums and drinking good beer. He lives near Manchester, UK, with his good lady and three beautiful children.

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