Dowloading a file tries to mount a network drive; when the network computer is offline, after a long time a message says "connection failed".

Try to delete the file downloads.sqlite in the Profile Folder

Help > Troubleshooting Information > Profile Directory: Open Containing Folder

Thanks for the suggestion. I deleted the file and restarted Firefox, but that did not fix the problem.

I ended up deleting my profile and creating a new one. That seems to work!

I solved this by clearing all history (Cookies, Cache, Browsing & download history, etc... everything.)

Attaching image documenting the problem.

Alot of problems like this sounds like a hi-jacking gone wrong. That's why clearing all history (Cookies, Cache, Browsing & download history, etc...) is a good idea all the time, many hi-jacking and tracking tactics used by advertisers rely on the fact few people set their browser to clear all private data on close. And a lot of redirects, corrupted address, and malicious stuff can cause weird network resource behavior