If firefox is configured to allow pages to choose their own fonts and you visit a uk government website (https://www.gov.uk/) the tab crashes.
If you configure firefox to disallow pages to choose their own fonts the tab works fine.
This has been happening for some time (at least 3 months, but possibly much longer). I just tried this with firefox-76.0_3,1 on FreeBSD 12.1 amd64 and with a brand new profile.
Note that with firefox for OS X this doesn't happen, so the issue appears to be specific to FreeBSD.
I am a very heavy user of firefox on FreeBSD and this only ever seems to happen for uk government websites, they seem to provide their own fonts.
Duplicate of bug 245915?
Yes, upgrading to fontconfig-2.13.92_2,1 fixed the problem.
thanks very much for your help.
*** This bug has been marked as a duplicate of bug 245915 ***