Download Firefox Developer Edition 139.0

New
- Full-Page Translations are now available within Firefox extension pages.
Fixed
- PNG images with transparency now keep their transparency when pasted into Firefox.
- The upload performance of HTTP/3 been significantly improved, particularly on resumed connections (QUIC 0-RTT) and high bandwidth and high delay connections.
- Fix for MFSA 2025-36 included in Firefox 139.0b10 and newer.
Developer

Web Platform
- window.getSelection().toString() now correctly returns the text serialization when text is selected in a text control, improving cross-browser interoperability on some sites.
- Closed <details> elements are now searchable and can be automatically expanded if found via find-in-page.
- Timer throttling for Workers is now supported.
- The Temporal proposal is now enabled by default in Firefox. Temporal is a better version of Date, for more details, please see https://spidermonkey.dev/blog/2025/04/11/shipping-temporal.html and https://tc39.es/proposal-temporal/docs/.
- Added support for the WebAuthn largeBlob extension.
- Added support for requestClose() to HTMLDialogElement.
- Service Workers are now available in Private Browsing Mode. This enhancement builds on our efforts to support IndexedDB and the DOM Cache API in Private Browsing through encrypted storage. With this change, more websites, especially those that rely on background tasks, will be able to benefit from Service workers.
- Firefox now supports the hidden=until-found attribute, allowing content to be found via find-in-page when it's otherwise hidden by default.
- The built-in editor for contenteditable and designMode now handles collapsible white-space(s) before block boundaries and white-space sequences between visible content more consistently with Chrome. As a result, Gecko no longer inserts a padding <br> element after white-space before a block boundary, aligning behavior with other browsers.