Sorry, I forgot to tell my O.s.: Ubuntu 14.04 x86-64. Thank you for the patience... :)

Hi, Separate Security Issue, I do not know what you use on linux to remove Flash, but you need to remove these : Flash 11.2 r202, Flash 20.0 r0, Flash 30.0 r0 as they are a Security issue of magnitude along with conflicting install. Need to remove 30 also as they get all hooked together. Re-install with Flash Player Version: 30.0.0.113 Current Version https://get.adobe.com/flashplayer/otherversions/ Step 1: Select Operating System Step 2: Select A Version (Firefox, . . . .)

It seems having a issue alright. Are you running any adblockers or security software that could interfere with this ?

Please  : Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

Pkshadow said

Hi, Separate Security Issue, I do not know what you use on linux to remove Flash, but you need to remove these : Flash 11.2 r202, Flash 20.0 r0, Flash 30.0 r0 as they are a Security issue of magnitude along with conflicting install. Need to remove 30 also as they get all hooked together. [...]

Thank you very much for your reply! I was able to fix the problem and succesfully install the new version of Firefox by removing old flash installations liying around in Ubuntu 14.04, doing the steps illustrated at the following link: https://askubuntu.com/a/815427

Then removing the symlinks to pipelight (if installed) from:

/usr/lib/mozilla/plugins

Then removing all flash plugins phisically remaining in: /home/[user]/.mozilla/firefox/[profile]/plugins

[...] Re-install with Flash Player Version: 30.0.0.113 Current Version https://get.adobe.com/flashplayer/otherversions/ Step 1: Select Operating System Step 2: Select A Version (Firefox, . . . .) [...]

... And now Firefox 61 works fine!

It seems having a issue alright. Are you running any adblockers or security software that could interfere with this ?

Yes: uBlock Origin, but it doesn't interfere.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

Here I am. Thank you again!