Sorry to hear you are having so many different problems. Let's start here:

e10s

One of the headline changes in Firefox 48-49 is e10s, which separates the browser interface process from the page content process. The performance impact of this can vary a lot between systems: many users find it faster, some find it slower, for many it's neutral. Hopefully by the time all the kinks are worked out it will work smoothly for everyone.

Are you using e10s?

You can check whether you have this feature turned on as follows. Either:

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, check the row for "Multiprocess Windows" and see whether the number on the left side of the fraction is greater than zero. If so, you are using e10s.

If you are using e10s:

To help evaluate whether that feature is causing this problem, you could turn it off as follows:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste autos and pause while the list is filtered

(3) Double-click the browser.tabs.remote.autostart.2 preference to switch the value from true to false

Note: the exact name of the preference may vary, but it will start with browser.tabs.remote.autostart

At your next Firefox startup, it should run in the traditional way. Any difference?

Iliya89 said

And worst of all, it comes with any interesting pastime (online game, watching videos, etc.) older versions this was not.

Flash??

Could you check whether you need an update to Flash? You can view your Flash version on my plugin lister page:

https://www.jeffersonscher.com/res/plugins.html

If Flash is not version 23.0.0.205, please update:

Flash Plugin - Keep it up to date and troubleshoot problems

Also, in case you did not see this article, please check the suggestions here:

Firefox hangs or is not responding - How to fix

I tried all the options over time. The problem is the same. Are there any other solutions to the problem?

Could you update us on the symptoms you are experiencing. For example, are some sites faster or slower, what messages do you get now, etc.

Yes, almost all the sites that have the option of viewing videos online. Ie all popular movie websites, and sites like Youtube. And the message is always the same: "Shockwave is busy or not responding". And the options: "Continue" or "force quit". And so - several times a day. Flash Player - the latest, 24th version. Shockwave is updated regularly. What could be the reason for such poor performance of Flash Player?

I don't know the answer to this:

What could be the reason for such poor performance of Flash Player?

However, two comments:

(1) When you checked for e10s, was it running or not running?

(2) YouTube uses the HTML5 player by default instead of Flash. Do you have that disabled in Firefox, either by hand-editing preferences in about:config or by an add-on that needs to force Flash to intercept/save/download videos?