This site works stupid. Lsat addon - mosilla archive fiormat. All this addons are tested by me, i use them many years. Screenchot not loads in question, now ill try to fix it:

Start '''[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Firefox in Safe Mode]''' {web Link} by holding down the '''<Shift><br> ''(Mac Options)'' ''' key, and then starting Firefox. Is the problem still there? ------------- Start your '''Computer''' in safe mode with networking. Then start Firefox. Try '''Safe''' web sites. Is the problem still there? '''[http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting The Computer In Safe Mode;<br>Free Online Encyclopedia]''' ---------- '''[https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-ram Firefox uses too much memory (RAM) - How to fix]''' This article describes how to make Firefox use less memory to make it run faster and prevent crashes. '''https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster''' '''https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix''' '''https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding'''

Looks like Adobe Flash is trying to play a video at YouTube. Have you tried selecting HTML5 for videos rather than using Flash? Have you checked for updated video / graphics drivers for that PC? https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration As far as Flash goes, have you disabled Hardware Acceleration for Flash? https://support.mozilla.org/en-US/kb/adobe-flash-plugin-has-crashed#w_disable-hardware-acceleration-in-flash There is a Hardware Acceleration setting in both Firefox and in Flash. Sometimes it is necessary to disable both