HI Mark

The extension you mention has not been updated as most of what it does is now included as standard within Firefox, plus there is a similar add-on available to fill in the rest, so no problems there. I suggest you create a restore point in XP and then upgrade to the latest Firefox and try it out for a week or so, then if you still don’t like it (Although you will wonder why you didn’t upgrade ages ago) you can easily re-install the old version, or if not available a simple system restore will bring it back.
Excel does more than just spreadsheets as it can even import Access files a page at a time, so no need to worry there.
I suggest you install Virtual Box on your Win 10 computer, and load XP into a virtual box which will allow you to run all your old software, if for some strange reason you cannot get it to run on Win 10. (A virtual box will allow you to run Win 10 (With all its security features) and XP simultaneously)
https://www.youtube.com/watch?v=_D9kYJ9KPlo&list=PLEADC4E42C8C89F07&index=168

Bill