That's not
the reason, but
the excuse. The reason Firefox doesn't have jxl is that it is funded by Google, and someone at Google decided that it has to die.
Also the parent comment was about that you really shouldn't just let a random Russian guy run any javascript on any website you visit, that's stupid.
Also also, am I missing something, or Firefox extensions are broken, there is no way to limit an extension to websites (allow or disallow), or even just to check the source code of an extension?