Tested on FF 78.6.1esr, Linux.
Attempting to view source (Ctrl+U) on any page with "Filter HTML Source Code" activated leads to a "Gah. Your tab just crashed." page.
Toggling Filter HTML Source Code off allows view source to work correctly.
Thanks for your report. This issue is known and it's a bug in Firefox, see #234. I've created a bug report at Mozilla. (https://bugzilla.mozilla.org/show_bug.cgi?id=1688146)
I did integrate a workaround for a short time, but it caused problems on some websites. That' s why I have removed it and am now waiting for Mozilla to fix it. It would be great if you upload the crash reports.
For anyone following up on this, FF duplicated the filed bug against https://bugzilla.mozilla.org/show_bug.cgi?id=1678734 . It also affects uBlock Origin and Livemarks.
They claim this is fixed in FF 86. I will note that 78esr is still affected.
Thanks for your help!
Deleting a branch is permanent. It CANNOT be undone. Continue?