Just a quick reference for those out there that encounter this problem. Today I was having problems with Firefox failing to save any pictures to my harddrive. The moment I clicked "Save as..." the screen would blink and there would be a "Failed' item under my downloads list.
I tried Refreshing Firefox and also creating a couple of new Firefox profiles. But when I tried to Refresh or create a new profile, I encountered this error message:
"This add on could not be installed because it appears to be corrupt."
I have never seen this error before. Soon all the add-ons from Mozilla I tried to install were seemingly corrupt. More and more digging finally lead me to an unlikely problem:
On my Linux machine, the /tmpfs drive (located in /run/user/1000) had completely filled up. This has also likely been the source of problems with Ktorrent not functioning properly.
So to those who encounter this problem: Clear your tmpfs! I rebooted and it solved the problem temporarily.