Does this prevent the video from playing, or does it lead to a 15-30 second gap when nothing appears to be happening, or... what happens exactly?

Firefox doesn't normally block ads, but if you are in a private browsing window, some sites are blocked by the Tracking Protection feature. A special "shield" icon should appear at the left end of the address bar in that case. Could you review the following article and see whether that is the issue: What happened to Tracking Protection?.

Since you mentioned seeing the same thing in Edge, could it be a system feature such as the family safety filter???

No youtube works with no problems but ads do not appear. I'm not in a private browsing window and not family safety filter. Only chrome works fine showing ads on youtube. microsoft edge and firefox not. Yesterday at a small period of time ads reappeared on youtube with firefox but when I closed the firefox window and reopened there were no ads again. I can't anderstand what is the problem. I also have installed kaspersky standard edition. I haven't found any option in it and I tried to see what happens by uninstalling it. Same thing again.

Where are the ads missing from? Do you mean video pre-roll ads or ads somewhere else on the page?

Everywhere when I open a video. No skip ads no side bar ads. Nothing. Like I am a youtube red member without being...

Well microsoft edge now show ads on youtube. I unchecked the disable of the pop ups there. After that I saw that ads were appearing again. I don't know if that was the problem in microsoft edge because I enabled it again and ads were appearing. So now edge and chrome work fine with ads. I tried to do the same on mozilla but the problem remains. I uninstalled and installed it again with no luck. I don't know which option blocks the ads. From the many times I open firefox the one ads appeared on youtube. But when I closed the firefox window and reopened no ads again. I can't understand where is the problem.