Quick update as promised. It's now been 5 days since Firefox itself was reinstalled and so far the extension has worked normally. I am under pressure from the support team to close the ticket but have asked for it to be left open until at least Friday so will provide a further update then.
Like here in the community where we ask you to mark something as a solution if it resolved your issue, we ask if a ticket can be closed because otherwise they can be forgotten and just sit there.
If you're not ready to close a ticket you can reply to it and say so.
I'm pleased to say that the extension has remained stable for over a week now so I've confirmed to LastPass that I'm happy for them to close my support ticket on the basis that the problem I reported is no longer occurring. If it does show itself again I will update this post but hopefully reinstalling Firefox followed by a fresh install of the extension will work for everyone else too.
Wouldn't you know it, by coincidence, it happened again. I was hoping from that last post by jamiesop1 that perhaps Firefox of LastPass had addressed the problem in recent updates, but no.