Personally, I don't think there's ever been a better time to start building an extension for Firefox. Some might disagree with me here.
So, Mozilla did drop old extension APIs, but I think you're misunderstanding things here. They didn't just drop, I don't know, the tab renaming API or something like that, instead they dropped the entire extension API. So, the entirety of how you used to write extensions for Firefox does not work anymore.
And they had two of those entire extension APIs (XUL/XPCOM extensions and Jetpack extensions), so yeah, they did drop extension APIs in the plural.
But there's now a new way to write extensions for Firefox, a new entire extension API, called WebExtensions. It's based on Chrome's extension API, so most Chrome extensions can now be ported to Firefox without much hassle. This also means that offering your extension for both Chrome and Firefox is now essentially half the work as before.
The new extension API is also just a lot simpler to work with and your extension will not anymore break with new Firefox releases, like they often used to in the past.
The new extension API is more limited than the old one, which basically is necessary to achieve the two previously mentioned points, so that's why some people might think that it's worse and that it's now not anymore worth to write an extension for Firefox at all and whatnot. They're sometimes a tad overly dramatic there...
Also, the new extension API isn't yet quite stable. The core of it, which was taken from Chrome, obviously is, but Mozilla is still working on more APIs to add to WebExtensions, so depending on what you're wanting to build, it might be a better to do that in a few months from now. But if you're just starting out with writing extensions, it's unlikely that you'll hit your head there right away.
Lastly, because of this deprecation of old extensions, there is a demand now for new extensions. There's gonna be a good number of niches that one can build extensions for and have people actually use them. It's not like we're starting out from zero, many extensions have already been ported (either rewritten or from Chrome), but yeah, still plenty of room, too.
So, Mozilla did drop old extension APIs, but I think you're misunderstanding things here. They didn't just drop, I don't know, the tab renaming API or something like that, instead they dropped the entire extension API. So, the entirety of how you used to write extensions for Firefox does not work anymore.
And they had two of those entire extension APIs (XUL/XPCOM extensions and Jetpack extensions), so yeah, they did drop extension APIs in the plural.
But there's now a new way to write extensions for Firefox, a new entire extension API, called WebExtensions. It's based on Chrome's extension API, so most Chrome extensions can now be ported to Firefox without much hassle. This also means that offering your extension for both Chrome and Firefox is now essentially half the work as before.
The new extension API is also just a lot simpler to work with and your extension will not anymore break with new Firefox releases, like they often used to in the past.
The new extension API is more limited than the old one, which basically is necessary to achieve the two previously mentioned points, so that's why some people might think that it's worse and that it's now not anymore worth to write an extension for Firefox at all and whatnot. They're sometimes a tad overly dramatic there...
Also, the new extension API isn't yet quite stable. The core of it, which was taken from Chrome, obviously is, but Mozilla is still working on more APIs to add to WebExtensions, so depending on what you're wanting to build, it might be a better to do that in a few months from now. But if you're just starting out with writing extensions, it's unlikely that you'll hit your head there right away.
Lastly, because of this deprecation of old extensions, there is a demand now for new extensions. There's gonna be a good number of niches that one can build extensions for and have people actually use them. It's not like we're starting out from zero, many extensions have already been ported (either rewritten or from Chrome), but yeah, still plenty of room, too.