I found the solution for this:

As it turns out, FF lost access permissions to the hard drive in which the %TEMP% folder was located. Changing the cache directory in about:config (browser.cache.disk.parent_directory) doesn't solve the issue, but changing the folder path in the ENVIRONMENT VARIABLES does.

Cheers

That was very good work. Well Done.

What does does this mean: "...but changing in the folder in PATH does"??? Please better explain your statement.

What does does this mean: "...but changing in the folder in PATH does"??? Please better explain your statement.

Hello Short answer: the path of the TEMP and TMP folders in the environment variables

Long answer: Firefox uses in windows (and I suspect in other OSs as well) the TEMP folder to store the files as they downloads. If you check your environment variables you will see that the TEMP/TMP folder are assigned to a hard drive that firefox can't write to because of permission restrictions. In my case the TEMP path was in D: and FF only had access to C: so I just changed the TEMP path to C: and it started working again. It's a hack since it can affect other programs and you are forced to change the temp folder but for now it works. A better solution is to give FF write acess to other hard drives somehow.

Note that current releases no longer use the OS temp folder, but download all files to the default downloads folder as set in "Settings -> General -> Downloads".

• https://support.mozilla.org/en-US/kb/manage-downloads-preferences-using-downloads-menu
• https://support.mozilla.org/en-US/kb/where-find-and-manage-downloaded-files-firefox
• https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file