Firefox is dead. You need an insurmountable amount of configuration to even make it bearable and there is non-user respecting settings and telemetry everywhere. Ads, too. It's not something you can recommend, every site is broken and Mozilla rather likes to spend it's money on [1] discouraging human translators and [2] giving people free coffee on "Browser Raves" in Berlin instead. It's a shadow of its former self.
it's the same kind of "workflow optimization" that notion and obsidian users suffer from. You spend so much time making your tools more productive but don't get any actual work done.
Firefox should commit more to correctly implement web standards - not even gradients render correctly. A lot of the users are oddballs with strange configurations that break everything. No wonder devs optimize for chrome.
google is a customer of fflabs and has enrolled them in summer of code. They also provide free fuzzing. ffmpeg is a foss, gpl-licensed project. nobody has any obligation to contribute, thus it isn't exploitation.
Why not just write an XSLT implementation in JS/WASM, or compile the existing one to WASM? This is the same approach that Firefox uses for PDFs and Ruffle for Flash. That way it is still supported by the browser and sandboxed.