The bugs I discovered yesterday have been somewhat mitigated. Basically, if I start Firefox with --ProfileManager and use a new profile then everything works properly within the browser; of course, that means I have to click an extra button every time I load the browser (annoying, not deadly), and I've lost all of my site logins and similarly useful browser state, but I can recreate all that quite readily.

However, just to irk me that little bit more, now Thunderbird has lost its connection to Firefox, so I can't click on emailed links to load the page (again). I gather this was a side-effect of the advice to unmerge and then re-emerge Firefox. Whatever little bit of magic I created way back when to pipe the two together must have been removed with the original installation :( .