Do you have NVidia’s network access manager or Forceware Network Manager software installed? Disable / remove it. https://support.mozilla.org/en-US/kb/connectivity-crashing-firefox-43-update

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. Is the problem still there?

Hi, FredMcD!

No, I don't have any of those installed.

I had tried Safe mode in all the browsers I tested, and it doesn't help with the issue.

https://support.mozilla.org/en-US/kb/websites-look-wrong-or-appear-differently

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

https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding

Thanks, I tried most of those, but with no effect.

• The problem with newer versions (38 and 43) showed on fresh installs; I didn't bother creating new profiles, but just reinstalled, which didn't help.
• Disabling hardware acceleration in Tools-Options has no effect.
• As for this step, I use RequestPolicy plugin, which blocks request (and downloads) from certain domains. It was active in all the mentioned version instances (older and newer ones).
• But still, disabling all the plugins and extensions - manually and via Safe mode - didn't help either.
• In fact, the only plugin that runs always is Cisco OpenH264 Codec, anything else is set to Ask to activate.

I suppose, it might be a bug in the end, considering lots of other reports for both Linux and Windows

Sorry, formatted links didn't work:

"this step" - https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster#w_block-unneeded-content

Linux high CPU usage report on Youtube HTML5: https://bugzilla.mozilla.org/show_bug.cgi?id=970308

Windows high CPU usage report on Youtube HTML5 (starting v37): https://bugzilla.mozilla.org/show_bug.cgi?id=1163327

Your comparisons and troubleshooting is all over the place; there's no baseline for judgement. And those Bug reports that you mentioned are worthless musings from users, IMO; more like support requests that were ignored by the developers because that's not what Bugzilla is meant for.

Try to confine yourself to Firefox 38 ESR and Firefox 43.0 (and Firefox 44.0 which is due to be released tomorrow). Past versions aren't supported any longer, and may only serve to confuse us all; and the level of HTML5 may be different between the various release versions.

And if you are using both Firefox 43.0 and the Firefox 38 ESR version, make sure you have a separate Profile for each and you only use the appropriate Profile for each version. This may or may not make a difference at times, but at other times using a Profile with different version might "really screw up the works". Yeah, Mozilla might look the same, until it's different underneath. And IMO, what Mozilla calls a "fresh install", isn't really "fresh" if you don't include a new Profile when trying to solve problems with Firefox.

Hi the-edmeister!

I use portable versions of FF mostly these days, that is why there is no need to manipulate the Profiles. Sure it's usually better to stick to the newest versions of software. But right now newest versions do have problems, critical enough for some people to downgrade. Which is not the question anyway; the older versions are in my post just to show they do ok.

The links are to show that lots of other people have the same issue, moreover irrespective to the platform. There are other similar complaints and reports, like FPS drops in Twitch's player with HTML5 controls, and similar to the subj., posting which I think is redundant.

And as for the steps I listed - they are in response to the links from the previous poster. I did try every general solution I could think of (and of course such obvious things as to clear cache, cookies and reboot) and mentioned than nvidia software is not the culprit, so that a person willing to reply might come up with some less obvious thing. Like maybe some tricky rendering setting in about:config, which might have been introduced/changed in the problem versions.