Flash Player not working suddenly

You can also use other products from Firefox such as Waterfox, Mozilla Thunderbird and so on, they don't have this lag

When things are weird, it's usually something pretty fundamental.  You're seeing *something*, so Flash is installed and working. 

The old saying "when in doubt, reboot" is still really good advice.  I'd actually power the machine down, let it sit for a few seconds so that everything fully discharges, then fire it back up. 

If the problem persists, clear your browser cache, maybe run the disk checker utility.  It's hard to tell from your description if it's a thing where things aren't getting drawn to the screen (i.e. something weird happening at the GPU driver), or if maybe it's a backend service problem and you're just not getting all the assets to load for some reason. 

I'd reboot.  That eliminates a whole universe of stuff we'd never be able to diagnose anyway.  Random cosmic ray crossed a billion light-years of space just to intersect with your computer in just the right way to make it run weird.  Split-second brownout.  Who knows.

At that point, at least we know that the problem space is fairly constrained and repeatable.  I'd try a couple browsers.  If it's limited to Firefox, you have a good idea that they dinged something.  If you see it everywhere, then I'd probably grab the latest video card drivers available, and then maybe disable hardware acceleration if that doesn't help (performance will suck, but if it starts rendering everything consistently, we know it's a GPU-related problem.  On the Flash side, that code is mature and rarely touched, so it's much more likely to be some combination of OS/Browser/GPU incompatibility).