If you reinstalled Firefox in a new program folder (renaming or deleting the old program folder) and tested in a clean new profile, and the problem remains, I wonder whether an external program such as a firewall or content filter might be blocking Firefox, or whether there is an issue with your connection, either direct through your ISP or using a VPN/Tor.

When Firefox becomes blocked or unable to load new pages, do other browsers still work normally for the same sites?

Thanks for the prompt reply! But no, the firewall's off, antivirus (Avira) set to allow Firefox. The ISP's fine (my wife isn't experiencing any connection/loading problems with her laptop; we're connected through a router). I tested another browser (IE) once, and it failed to load (it worked when Firefox worked). Any other suggestions -- like, should I uninstall and try to find an earlier viersion of Firefox?

There were a few reports like this for Firefox 36, but then I saw one on Firefox 37, but I didn't see any others until your report today. It's not possible for me to read everything, so I may have missed some, but I think it's an uncommon issue that isn't specific to Firefox 40.

If you find that other browsers have the same problem at the same time, it could be that a networking component of Windows "died" in memory or is misconfigured.

You could look into these Windows tools to see whether they help:

• Use the System File Checker tool to repair missing or corrupted system files

• How to reset TCP/IP by using the NetShell utility

Again I thank you for the fast reply and suggestions! Ran the File Checker, got "Windows Resource Protection did not find any integrity violations." Ran Microsoft's FixIt to reset TCP/IP and it said all was fixed. Told to re-boot, so I did; ran Firefox to get my Gmail, and page loaded OK; opened several new tabs and pages opened OK. Now I guess I'll have to wait until tomorrow to run Firefox for more than an hour, to see if it dramatically slows or fails to load pages? Will advise by mid-morning, if you're interested. Thank you again!

P.S. If all OK, I'll very happily also click "Solved the problem"!

Congratulations, jscher2000! Made it through the day with no problems.