• Tools > Options > Security: Passwords: "Remember passwords for sites"

• Make sure that you not run Firefox in (permanent) Private Browsing - Use Firefox without saving history mode.

• You enter Private Browsing mode if you select: Tools > Options > Privacy > History: Firefox will: "Never Remember History"
• To see all History and Cookie settings, choose: Tools > Options > Privacy, choose the setting Firefox will: Use custom settings for history
• Uncheck: [ ] "Automatically start Firefox in a private browsing session"

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the add-ons is causing the problem (switch to the DEFAULT theme: Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.

See:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Many thanks. There is no such option as "Automatically start Firefox in a private browsing session" in my version??

I think I have managed to sort it out anyway by changing from "Never Remember History" to" Remember History". It seems to work now

In Firefox 4 it is named "Permanent Private Browsing mode", but you still need to select "Use custom settings" to see that check box. If you select "Never remember history" then that PB mode check box will be checked automatically and selecting "Remember history" like you did will reverse it and uncheck the permanent PB mode setting.

Ah...I understand it now. Many thanks!

You're welcome