Mozilla tracking bug
I don't know the current status of Blahtexml, but the David Harvey had reported many Mozilla bugs in the past. There is a list here
https://www.mediawiki.org/wiki/Extension:Blahtex/Bugs/Bugs_in_browser_MathML_support/Gecko
and a quick search shows that many are still referenced in the source code:
https://github.com/gvanas/blahtexml/search?q=mozilla https://github.com/gvanas/blahtexml/search?q=firefox
The Mozilla MathML team has fixed many of these bugs (Blahtexml should probably be tested with a recent version of Gecko and the comments in the source code updated) but we are always interested to get more feedback:
https://bugzilla.mozilla.org/buglist.cgi?f1=see_also&list_id=11167342&o1=substring&query_format=advanced&v1=https%3A%2F%2Fgithub.com%2Fgvanas%2Fblahtexml%2Fissues%2F4
I'm opening this as a tracking bug.