Firefox DevTools Newsletter — 125

Posted by

Developer Tools help developers write and debug websites on Firefox. This newsletter gives an overview of the work we’ve done as part of the Firefox 125 Nightly release cycle.

Firefox being an open source project, we are grateful to get contributions from people outside of Mozilla, like Artem Manushenkov who updated the Debugger Watch Expressions panel input field placeholder (#1619201)

Want to help? DevTools are written in HTML, CSS and JS so any web developer can contribute! Read how to setup the work environment and check the list of mentored issues

Pop it up!

Firefox 125 adds support for the Popover API, which is now supported across all major browsers 🎉. As said on the related MDN page:

The Popover API provides developers with a standard, consistent, flexible mechanism for displaying popover content on top of other page content. Popover content can be controlled either declaratively using HTML attributes, or via JavaScript.

In HTML, popover elements can be declared with a popover attribute. The popover can then be toggled from a button element which specifies a popovertarget attribute referencing the id of the popover element.

Firefox DevTools Inspector markup view. We can see a button with a popovertarget attribute and next to it a "select element" button. A div element with a popover attribute is displayed as well
Inspector displayed on https://mdn.github.io/dom-examples/popover-api/blur-background/

In the Inspector markup view, an icon is displayed next to the popovertarget attribute so you can quickly jump to the popover element.
Popover element can be toggled in Javascript HTMLElement.showPopover, HTMLElement.hidePopover and HTMLElement.
togglePopover. beforetoggle and toggle elements are fired when a popover element is toggled, and the Debugger provides those events in the Event Listeners Breakpoints panel.

Note that we don’t display ::backdrop pseudo-element rules yet, but will be soon (target is Firefox 127, see #1893644)


As announced in the last newsletter, we’re focusing on performance for a few months to provide a fast and snappy experience to our beloved users. We’re happy to report that the Style Editor panel is now up to 20% faster to open (#1884072).

Chart where x is the time and y is duration, where we can see the values going from 750ms to 600ms around March 14th
Performance test duration going from ~750ms to ~600ms

We also improved the Debugger opening when a page contains a lot of Javascript sources (#1880809). In a specific case, we could spend around 9 whole seconds to process the different sources and populate the sources tree (see the 124 Firefox profile). In 125, it now only take a bit more than 600 milliseconds, meaning it’s now 14 times faster (see the 125 Firefox profile).

Firefox Profiler Flame chart screenshot for the same function, on Firefox 124 and 125.

This also shows up on less extreme cases: our performance tests reported an average of 3% improvement on Debugger opening.


There is now a button indicating if the opened file is an original file or a bundle, or if there was an issue when trying to retrieve the Source Map file (#1853899).

Firefox debugger with a tsx file opened.
At the bottom of the file text, there a button saying "original file". A popup menu is opened and has the following items:
- Enable Source Maps
- Show and open original location by default
- Jump to the related original source
- Open the Source Map file in a new tab

Clicking on the button opens a menu dedicated to Source Map, where you can:

  • enable or disable Source Map
  • indicate if the Debugger should open original files by default
  • select the related original/bundle source
  • open the .map file in a new Firefox tab

We also fixed a glitch around text selection and line highlighting (#1878698), as well as an issue which was preventing the Outline panel to work properly (#1879322). Finally we added back the preference that allows to disable the paused debugger overlay (#1865439). If you want to do so, go to about:config , search for devtools.debugger.features.overlay and toggle it to false.


  • CSP error messages in the Console now provide the effective directive (#1848315)
  • Infinity wasn’t visible in the Console auto-completion menu (#1698260)
  • Clicking on a relative URL of an image in the Inspector now honor the document’s base URL (#1871391)
  • An issue that could provoke crashes of the Network Monitor is now fixed (#1884571)

Thank you for reading this and using our tools, see you in a few weeks for a new round of updates 🙂

Leave a Reply

Your email address will not be published. Required fields are marked *