You Tube Player Size Too Big in Firefox by default

Question

I recently experienced that the You Tube player automatically became large.
The standard size for YouTube videos seems to have changed from their old standard into the size that used to be halfway between normal and fullscreen.
That is WAY too big, and it's actually disgusting to look at with 360p and below videos. The vast majority of videos look like crap when they're stretched out.
There is a new button, "centered player," where the old "large player" button used to be. Clicking on it will center my enormous window, then that button displays "default player," which brings me back to this large, uncentered player.
This is happening with Firefox, with IE working fine.
Although I am technically incompetent I tried a few suggestions:
Remove all cookies
Hit Ctrl+Shift+K and insert the code below in console:
document.cookie="VISITOR_INFO1_LIVE=; path=/; domain=.youtube.com";window.location.reload();
However, I am getting the following messages:
Use of getAttributeNodeNS() is deprecated. Use getAttributeNS() instead. util-service.jsm:590
Use of getPreventDefault() is deprecated. Use defaultPrevented instead.
And it is not working, back to same Large Player….
Hopefully Mozilla can do something to fix this.
Thanks!

Answer 1

You said that you removed the cookies. Now
* Clear the Cache
'''[https://support.mozilla.org/en-US/kb/how-clear-firefox-cache?redirectlocale=en-US&redirectslug=clear-cache-delete-temporary-files-fix-issues How to clear the Firefox cache]''' {web link}
If there is still a problem,
'''''[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Start Firefox in Safe Mode]''''' {web link}
While you are in safe mode;
Press the '''<Alt>''' or '''<F10>''' key to bring up the tool bar.
Followed by;
Windows; '''Tools > Options'''
Linux; '''Edit > Preferences'''
Mac; ''application name'' '''> Preferences'''
Then '''Advanced > General.'''
Look for and turn off '''Use Hardware Acceleration'''.
Poke around safe web sites and see if there is still a problem. Then restart.