Hi Kimberl, You wouldn't happen to be using AdGuard for Windows? I had a similar problem a couple of months ago which was caused by it.

If so, your options are to turn off guard for Firefox or uninstall Adguard.

Hi No, I do not have that plugin installed.

Kimberl, Take a look at this thread, see if it helps: Master password pop-up appears every browser start-up

Alas no, I am using a master password so do want a single pop up box.

I failed to mention this has been like it a couple of years now and a couple of PC's as well so reinstalls have not changed anything.

Hi Kimberl, I wish I had your level of tolerance. Have you tried temporarily disabling your security software? And, if you are using Sync, have you tried signing out?

Hi

I did not actually have any security installed for some time at one point. I'll try the signing out of sync.