I highly recommend you run a malware scan on your computer based on the myriad of extensions you are using in Firefox. This behavior can be indicative that malware may be running on your computer.

I recommend downloading malwarebytes free version and running a scan. Link: https://www.malwarebytes.com/

I also recommend completely removing all extensions you are not using(not just disable). Link: https://support.mozilla.org/en-US/kb/disable-or-remove-add-ons#w_how-to-remove-extensions-and-themes

As a side note...I would exercise more caution/restraint when choosing to install third party extensions in Firefox. Give the article below article a read when you have a moment. Link: http://www.cmswire.com/information-management/firefox-extensions-are-malware-magnets/

Thanks Fiz for your reply. I learned a lot from the link your provided talking about third party extensions. I really had a lot, about 30+. Now I'm using only 9 that I mostly use everyday.

I did what you suggested, installed Malwarebytes, ran a scan, but nothing weird was found, no warnings or suspicious files.

I expected nothing to be found, since I consider myself a person with a fair knowledge of how not to get malwares, so I don't browse suspicious websites, nor open any attachments from untrusted people, and even the ones that come from trusted people, I don't open them without my antivirus checking it first.

I didn't know about the breaches from my browser extensions, so I'm gonna keep a close eye on them as well, but having my computer with antivirus, firewall activated, safe browsing and careful with e-mails I get, I hoped not to get any warnings as it happened.

Well, there was one thing I hadn't done before opening this problem. Although I know it should have been my first try, I hadn't restarted my computer. I know it is the basic solution for a problem in a Windows machine, but you know, I'm so used to only hibernate my computer, that I sometimes forget about this simple solution.

I restarted my computer and the issue stopped happening. I still don't know what was the cause for this weird issue that only happened for Firefox. All other browsers or applications were working just fine for saving.

It has been two days since the issue hasn't come back, so I think it won't be back anymore (hopefully). But I'll update here if it does.

Thanks once again for you post.

The website you were accessing, was it in your default language?

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?