H1 user ryotac reported yet another variant of https://gitlab.torproject.org/tpo/applications/tor-browser/-/issues/30683, but this one is not covered by that patch because XSLT errors have their own separate bundle and displaying logic. POC: https://people.torproject.org/~ma1/pbugs/42288/poc/ ~~I believe in this case, rather than spoofing English in the error page, we should just match Chromium's behavior and just fail silently, displaying the untransformed document - and possibly fix upstream too, [looking at this comment](https://searchfox.org/mozilla-central/rev/42acdc9cd5ae89222bdceeeaed7bacac755be48f/dom/xslt/xslt/txMozillaXSLTProcessor.cpp#490).~~ [EDIT] In the end I decided to fix this with logic similar to #30683, instead of radically changing Firefox's behavior (potentially informative and valuable to developers debugging XML/XSLT problems) to align it with Chromium. So in !851 we rather spoof language in XSLT error messages as well, when needed.