Are you using Tools > Options > General > Startup: "When Firefox Starts": "Show my windows and tabs from last time" to preserve your tabs or do you use the Save & Quit dialog?

The first is much more reliable and you shouldn't lose open tabs.
See also http://kb.mozillazine.org/Session_Restore

I think you think my problem is between sessions (as it happens, as I seem to remember loosing tabs between sessions ALSO, my unwieldy solution to that has been to use Task Manager to kill FF, or just logoff my pc with it still open - and then I do get back my open tabs, maybe after a FF opening prompt to do so), but my question is my Open Tabs are lost WHEN I do an upgrade of the FF4betas - which has put me off doing any upgrades to my FF3.6 on my Notebook, until I know how to 100% secure the many open tabs I use for work-in-progress. BTW, in FF4 (have been using Chrome instead of the FF3.6 on the Notebook due to speed) I do not see a Tools > Options > General > Startup: "When Firefox Starts": "Show my windows and tabs from last time". In the top left corner of FF4 I see a red FF btn - when clked shows Options ... but only for various menu bars & Tabs on Top. Sorry about not being that familiar with FF - I moved to Chrome 2 yrs back due to poor speed on my weak Notebook, but miss a few useful Addons, and having read FF4 is now comparable in speed to Chrome, have been playing with FF4betas a little, with view to using FF4 a lot more.

You shouldn't use the Task Manager to kill the Firefox process unless you can't close Firefox normally. Using the Task Manager or otherwise forcing Firefox to close can always have side effects and possibly cause fie corruption.

You can make a backup copy of the file sessionstore.js in the Firefox Profile Folder to be sure that you do not lose that session.

As you appeared to think my question was about preserving my open-tabs between sessions of the same version of FF, rather than between FF version 'upgrades', as an aside I just mentioned the 'unwieldy' solution I use for intersessions. I know I should not have to kill a process to preserve its state for next time - but it works reliably so I use it.

The problem I need a solution for is: how do i preserve my open tabs when updating to a new version of FF ?

As I will not upgrade my FF3.6 on my Win XP notebook until I can prove a reliable solution on the Win 7 PC, I will limit my question to the Win 7 & FF4b environment. I started with FF4b4, currently have FF4b11 installed. Each time I tried the 'next version' of the beta, as I didn't immediately see any install notes on the FF4beta page, I simply clicked on the downloaded new .exe and let it do its install. It did NOT ask me if I wanted to preserve my existing settings, OR indeed make any reference to an existing installation. The only thing I noticed after the new install, is that my existing bookmarks were still there (maybe 'cause I subscribed to 'FF Sync').

Should I have seen an 'install note' somewhere, to guide my install of new version so it preserved my open tabs (and any other custom settings) ?

Ideally, your answer should work for all upgrades of FF, both 3.6 and FF4x

BTW my reply in response to your question: am I using Tools... said "I do not see a Tools>Options>General>Startup....." - is this, or something like it, relevant to my problem ?

Thanks

See also duplicate thread https://support.mozilla.com/en-US/questions/791458 I commenced answering that thread before realising it was a duplicate.

BTW Press Alt on the keyboard to temporarily display your menubar, which in XP includes the Tools menu, that will also work in Firefox 4RC1 even with the new Firefox Button displayed.