Seeems like image didnt attach.

That chrome:// file most likely is part of Firefox's program files and not so easy to edit (not to mention, subject to replacement at each update). In principle, it would be easier to override the built-in function with an extension or possibly a userscript. I haven't searched for one...

Any tips how to do it myself? Also is there any pettition to mozilla so they would change it to more default setting :)? Seems like it's just 10 sec to find and edit it, or maybe i can just compile full mozilla project with only this setting changed.

Nvm, did it myself.

Please explain what you did so others can try your idea.

I did exactly as @jscher2000 said, i just wrote an easiest extension that overrided the file with my edited file that had a changed value of time skip. I used this guide: https://developer.mozilla.org/en-US/docs/Building_an_Extension

you type "chrome://global/content/bindings/videocontrols.xml" in adress bar and you save it to get a xml file, then you open it with notepad, search for "oldval - 15" and "oldval + 15" and change it to whatever you want, put it in right folder (where sample.xul is in guide),

content of chrome.manifest: " content sample chrome/content/ override chrome://global/content/bindings/videocontrols.xml chrome://sample/content/videocontrols.xml "

i think that's the easiest way, i was planning to try to put this setting in about:config but it's working fine right now so i didn't bother.