I'd be really grateful if anyone has any ideas (and hopefully solutions) as to why this is happening.

Firefox has suddenly started deleting my cookies whenever I close the browser. I haven't changed any settings as far as I know. I checked the privacy options and "accept cookies" and "keep them until they expire" are both still checked. I've tried going back to a previous restore point but that didn't help and I've checked for spyware etc. using Ad-Aware and Spy Sweeper and while those two programmes got rid of some suspicious content, it didn't solve the problem. I'm using Firefox 3 which I installed not long after it was released so I've been using it quite a while now with no problems.

Can anyone help please?

I've done a little digging and have come across a thread with people posting that they have the exact same problem. Turns out it's something that happens to some people when they update to Firefox 3.0.1

Anyway, cor-el suggested the following...

Try to delete cookies.sqlite (and cookies.txt) in the Profile Folder

See Profiles: How to find your profile
"Application Data" in XP/2K and "AppData" in Vista are hidden folders, use %APPDATA%
Show hidden files and folders (MozillaZine (KB))

...which seems to have worked for me. So far so good anyway!

instead of an upgrade, try a clean install.

Check this setting in Firefox please:

Tools> Options> Privacy section> Private Data> Is there a check in 'Clear Private Data when I close the browser'?

If there is, uncheck it and the Cookies won't be deleted-OR-click on the Private Data settings, uncheck whatever you don't want removed IF you want the other Private Data checked.

I use FF v3.0.1 and have Cookies unchecked to be cleared.