Transferring the the Firefox forum. I assume it has nothing to do with Thunderbird.

I don't think so but I really don't know for sure

Hi, please check your pop-up blocker settings - Pop-up blocker settings, exceptions and troubleshooting.

Also try an ad blocker like Adblock Plus.

If your question is resolved by this or another answer, please take a minute to let us know. Thank you.

If you think you are being targeted,

You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

Thanks to everyone for their help, I hope that my problem has been solved

Stropping the popups needs removing the adware and everything associated. You might as well check thispopup removal guide.