What is the web site adddress and the file you are trying to download?

Start Firefox in Safe Mode {web link} While you are in safe mode;

Type about:preferences<Enter> in the address bar

Select Advanced > General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites. Are there any problems?

Then restart.

This pertains to the file -> UPLOAD <- dialogue.

... !!! upload -> Upload -> UPLOAD !!! ...

What is it that happens, error messages, anything else?

As per the original ticket: As soon as the kmozillahelper file system dialogue pops up, sometimes immediately, and sometimes after several seconds, Firefox freezes permanently. I have to kill the process and restart Firefox to proceed.

Additionally: There are no error messages of any kind. Neither in the GUI, nor in /var/log/messages. Nada, nix, just everytime I try to upload a file, Firefox freezes permanently (until I kill + restart Firefox).

Sadly :(

I guess the 'kmozillahelper file system dialogue' is the window which pops up after pressing a 'Browse' button in order to select a file from the file system? It's more likely that this is an OS problem, where the OS is trying to query destinations not immediately available, e.g. like a network share, or a Bluetooth service, as described in one of the links you posted above.

Does this problem only occur with Firefox? Have you tried a different browser, or do you see this with other applications as well?

Yes, Christ1, I can browse the file system for a file/photo to upload. Although only for a few seconds, and then Firefox freezes. It's very odd.

I agree that it sounds like some interaction with the firefox/kmozillahelper/sys. app. thingy. I have tried a clean chrome download, and the identical file upload, (site/image combo), works perfectly from there...

Interestingly... using a different user account with the same Firefox installation works fine. *also* using a different user PROFILE with the same Firefox installation works fine too. What I do is this:

 firefox --new-instance -Profile_Manager

profile-1 works fine. profile-2 fails with file upload browser window freeze. profile-3 works fine. profile-4 works fine. profile-5 fails with file upload browser window freeze. profile-6 works fine.

...

What is it about 2 and 5 that is differeent from the others? Add-on, setting, permissions . . . .

Just curious, where did you pick up the --new-instance command line argument? It isn't very well documented, if at all. Typically you'd use the -no-remote option to start a new instance. http://kb.mozillazine.org/Command_line_arguments

Let's not go off-topic here, @christ1, but I refer you to the official bug report which discusses "Create a -new-instance flag for the functionality of the old -no-remote flag."

 https://bugzilla.mozilla.org/show_bug.cgi?id=716110

It seems like which flag you choose to use, might depend on which bug report you read ;)

ps. inspecting profile differences...