Simple, they switched to the webextension API a few years ago as it makes development of extension across browsers much easier.
More directly put, developers would develop extensions for Chrome and more often than not would not bother with Firefox as the framework it used was different. By switching to the same framework for developing extensions it now means that it takes little to no effort for developers to also publish it for firefox.
Having said that, Firefox is providing compatibility with manifest v3 while at the same time not implementing some of the restrictions that are in place in the Chrome implementation.
> Simple, they switched to the webextension API a few years ago as it makes development of extension across browsers much easier.
Great idea to prioritize interop when the people who you're trying to interop with are gulping large chunks of your userbase. Very important to make sure that Firefox had parity with Chrome in extensions, the only area where Firefox previously had a massive advantage over Chrome.
> the only area where Firefox previously had a massive advantage over Chrome.
It didn't though, because the API was different a lot of extensions that appeal to a broader user base were not made available on firefox (anymore). Yes, the previous framework did allow other use cases more suitable for power users but those are a fraction of the total user base.
And in general it hugely simplified extension development. Instead of having to maintain various code paths for different browsers and accounting for slightly incompatible APIs this was no longer needed.
> More than 90 per cent of Mozilla's funding comes from web search providers that pay for the right to be the default search engine in Firefox in their regions. According to the organization's latest financial figures, $430m of its 2018 total revenue of $451m came from those internet giants – primarily Google
