Firefox 121 for developers
This article provides information about the changes in Firefox 121 that affect developers. Firefox 121 is the current Beta version of Firefox and ships on December 19, 2023.
Changes for web developers
Developer Tools
HTML
-
Lazy loading of
<iframes>
is now supported, allowing developers to hint that particular<iframe>
s should only be loaded when (and if) they become visible. This can speed up initial load time by reducing the resources that need to be fetched on page load (some<iframes>
may not need to be fetched at all). The hint can be provided via theloading
attribute on the<iframe>
element, or in JavaScript usingHTMLIFrameElement.loading
. (Firefox bug 1622090).
Removals
CSS
- The
:has()
selector, commonly referred to as the parent selector, is now supported. It allows developers to apply styles to an element based on related elements, e.g.li:has(ul)
will match a list which contains a sub-list, orh1:has(+ p)
will match a heading with a paragraph directly after it. - The
text-indent
CSS property now supports theeach-line
andhanging
property values (Firefox bug 784648), allowing developers to more easily specify certain text indentation styles. Additionally, developers can now combine multipletext-indent
property values for more creativity, e.g.text-indent: 3em hanging each-line
.
Removals
JavaScript
Date.parse()
now accepts several additional date formats:- Year > 9999 for
YYYY-MMM-DD
format (e.g.19999-Jan-01
) (Firefox bug 1858851) MMM-DD-YYYY
(e.g.Jan-01-1970
) (Firefox bug 1863489)- Milliseconds for non-ISO date formats (e.g.
Jan 1 1970 10:00:00.050
) (Firefox bug 1863125) - Day of week at the beginning of formats which were being rejected, such as:
Wed, 1970-01-01
Wed, 1970-Jan-01
foo 1970-01-01
works (Firefox bug 1617562).
- Year > 9999 for
- Other
Date.parse()
fixes:YYYY-M-DD
andYYYY-MM-D
are no longer assumed GMT as an ISO dateYYYY-MM-DD
would be (Firefox bug 1783731).- Milliseconds for all formats are truncated after 3 digits, rather than being rounded (Firefox bug 746529).
Removals
SVG
Removals
HTTP
Removals
Security
Removals
APIs
- The
sendOrder
property of theWebTransportSendStream
interface is now supported, allowing the relative priority of outgoing WebTransport API bidirectional streams to be read and changed after the stream is created. Note that this cannot be used to modify the priority of unidirectional streams, as Firefox implements them asWritableStream
instead ofWebTransportSendStream
. (For more information see Firefox bug 1853444.)
DOM
Media, WebRTC, and Web Audio
Removals
WebAssembly
Removals
WebDriver conformance (WebDriver BiDi, Marionette)
General
WebDriver BiDi
Marionette
Changes for add-on developers
Removals
Other
- Fixed a bug that resulted in background pages not starting when a manifest.json
background
key containsservice_worker
andscripts
declarations (Firefox bug 1860304).Note: Incidentally, a change in Chrome 121 sees the
scripts
property ignored when specified with theservice_worker
property. Previously, Chrome refused to load extensions containing both properties (Chromium bug 1418934). For more information, see Browser support of thebackground
manifest key.
Older versions
- Firefox 120 for developers
- Firefox 119 for developers
- Firefox 118 for developers
- Firefox 117 for developers
- Firefox 116 for developers
- Firefox 115 for developers
- Firefox 114 for developers
- Firefox 113 for developers
- Firefox 112 for developers
- Firefox 111 for developers
- Firefox 110 for developers
- Firefox 109 for developers
- Firefox 108 for developers
- Firefox 107 for developers
- Firefox 106 for developers
- Firefox 105 for developers
- Firefox 104 for developers
- Firefox 103 for developers
- Firefox 102 for developers
- Firefox 101 for developers
- Firefox 100 for developers
- Firefox 99 for developers
- Firefox 98 for developers
- Firefox 97 for developers
- Firefox 96 for developers
- Firefox 95 for developers
- Firefox 94 for developers
- Firefox 93 for developers
- Firefox 92 for developers
- Firefox 91 for developers
- Firefox 90 for developers