Comment by LunaSea

Firefox used to be the first browser with decent developer tools thanks to the Firebug extension.

Then Chrome pushed heavily the development of its own development tools and crushed Firefox in the process. Chrome has now been the best browser for developers for the last +10 years.

The consequence is also that when the new generation of headless browsers were developed (Puppeteer) they were based on Chromium because it is so hackable and developer friendly.

This means that Firefox lost a big chunk of the developer community which constituted also a non trivial amount of their user base and advocates.