You might want to read this:
http://support.mozilla.org/en-US/kb/changes-toolbars-and-window-sizes-are-not-saved
Thanks for the suggestion I was so hoping it was going to work but I've, literally, just finished doing a Firefox Reset as described (not as much hassle as I expected either). Customised the toolbar back to what I wanted, enabled the three Add-Ons I use one at a time, restarting Firefox after each one and leaving the Avast! Online Security 8.0.1488 until last.
All the other toolbar customisation - no problem, all remembered after each restart. Avast Online Security 8.0.1488 with WebRep - nah!
It behaves just as I originally described reverting back to the right of the search box on every Firefox restart. Disappointing.
As said, I didn't think it was a Firefox problem; the problem began immediately after the updated Avast Add-on installed. Pretty much a smoking gun I think.
It must be a specific issue for Avast with Firefox though. As mentioned I've been playing around with Opera recently and once I'd found out how to do it I customised the toolbar to the same layout as I use with Firefox. The WebRep icon stays exactly where I put it on the left. I haven't tried with EI but I don't use it very often and the default position for the Avast WebRep icon is on the left anyway.