Is this the add-on you tried?

Video Background Play Fix https://addons.mozilla.org/android/addon/video-background-play-fix/

I don't see any problems in the recent reviews...

is there other way to solve this? play background fix isn't compatible

This extension should be compatible with Firefox for Android. Can you double-check that you are not requesting the desktop site (which does not appear to be a mobile browser to the add-ons site)?

Hi i double checked it and still not compatible and the request desktop site is not checked. Btw i think it may help if i say that im using vivo y53

Hmm, I wasn't able to get it through

menu > Add-ons > Browse all Firefox Add-ons

However, I was able to install it by doing a web search and going directly to the page. ???

I jist paste this link u gave me https://addons.mozilla.org/android/addon/video-background-play-fix/, and i cant click the install button and there's a msg saying ur phone isnt compatible ~something like that

I don't know why you are getting that. Can you install any add-ons from the site at all? For example:

https://addons.mozilla.org/android/addon/videospeed/

I already did installed few add-ons loke video speed, last pass, adblock plus