It seems strange to reply to my own problem, but I'm doing it in the hope it might help someone who's in a fix, as I was. First, let me say that the problem of Firefox 4 being jammed may not have been the fault of firefox; it might have been this cranky vista operating system... but if you're introducing a browser for use on a cranky system, you need to tailor it to that system.... After I posted this problem, I tried a system restore to just before the firefox 4 download, which re-introduced firefox 3.6 on my programs, but it was still jammed and blocked. .... Then a friend recommended a free download: Revo Uninstaller, which will force uninstalling if necessary; and after an initial protest, Revo got to work and removed all the firefox files.... Then I re-downloaded firefox 3.6... and amazingly: it works! Even my tabs and bookmarks have been preserved, although I don't know how!.... For the future: if mozilla can't overcome this problem before they force firefox 4 on their users, then if it jams again, I have been advised to go to Google Chrome- which I don't want to do.... So, do, please, Firefox, give us a fully workable and user friendly system before you force us to go over to it [Firefox 4, that is]. Thank you so much, in anticipation.

You get a message to reboot if the updater notices that some files are in use by other programs and can't be replaced by the new version. A possible cause is security or other software that uses DLL in the Firefox program folder or locks other (.ini) files. It usually works if you do a clean install in such cases and remove the Firefox program folder manually or use the Revo uninstaller like you did.

Thank you very much cor-el. I greatly appreciate your wisdom which I may well need when the time comes that I cannot any longer prevent firefox from downloading version 4!