In extra support for this:-
I found a procedure posted in the thread (also regarding the need to reinstall the FF add on after every reboot) started by cel01 in this SiteAdvisor section of the forum.
This involved uninstalling SA (i.e. this is the one which IE installs) and then reinstalling the add-ons again, first from FF and from IE. The result of my doing this was to get me back to the same situation again - that is to say it did not cure the problem for my system.
However, on the way I found that if SA is uninstalled so that it is not available in IE then in FF there is no unwanted item, only the (26.6) add-on installed using FF. This now works without any problems, it is still there after a reboot and bookmarks/history still remain after using the BBC iPlayer (Adobe Flash) when FF is closed and reopened.
So this means I have two choices:-
1. Uninstall (or don't install) SA for IE and have SA working properly in FF, but of course not then available in IE.
2. Have SA in IE but make sure the item that this installs in FF is disabled and reinstall SA from FF after every reboot.
This I consider to be evidence that the installation of SA from IE is unsatisfactory and that it is causing problems with the FF add-on.
I had intended to call this post 'SA for IE could need some attention from McAfee' - but I pressed the wrong button so it finished up just as 'S'.