Client side XSLT transforms are very rarely useful. It was intended for a vision of the web as a multiude of different XML formats served to end users. Since this didnt happen (for good reasons), XSLT becase less useful. In reality it is only ever used to tranform into XHTML.
It was also touted as a more powerful style sheet since it could rearrange elements and transform semantic elements into tables for presentation. But CSS supports that now in a much better way, and most significantly CSS can react to dynamic updates.
Serving XML to end users only makes sense if someone actully understands the XML format. But only very few XML formats beside XHTML and SVG have any broad support. RSS is one of the few examples, and seem to be the main use case for XSLT.
Gecko currently has much deeper integration of the XSLT engine with the browser internals: The XSLT engine operates on the browser DOM implementation. WebKit and Chromium integrate with libxslt in a way that's inherently bad for performance ( https://github.com/whatwg/html/issues/11578#issuecomment-321... )
Just Firefox XSLT is faster, better, cheaper than Google's (and JS), same, old Firefox extensions were to powerful Google could compete with Firefox (or block adblocks).
JS is very needed for ads, tracking and other strings attaching - and XSLT is not for that - but would make JS mostly obsolete in many cases.. (only "cross-browser functionality for XSLT is incomplete with certain features like <xsl:text disable-output-escaping="yes"> having open issues" ).
Google pay Mozilla to criple Firefox. It's money from ads, to not let the web be free. Right now, how much $ and CPU power a JS engine could cost, for that, is irrelevant - except for the final user !