Hi, yes released today 59.0.2 https://support.mozilla.org/en-US/kb/update-firefox-latest-version Fixed issue that when Cleartype Font in Windows was not enabled it would break webpages.

Please update, Clear Cache and cookies and would hope your fixed.

Please let us know if this solved your issue or if need further assistance.

Hmmm, I cleared everything when it came to the cache and cookies and then manually updated firefox but that doesn't seem to have fixed the issue, is it possible the update isn't out in my region or something?

Just to update, apparently the problem is also appearing on microsoft edge based on the post in the Google forums, so its probably not a firefox issue, just a coincidence that the first people to post all used firefox I guess, thanks anyway! I'll post back when if its fixed for everyone but firefox users

A style rule in the page is suppressing the scroll bar. I don't know why there are different results in different browsers; possibly Google sends slightly different rules. ??

I posted a workaround in your Google Product Forums thread, but I'll repeat it here.

This custom style rule will strip off the problem rule:

#guide-inner-content.ytd-app {
  overflow-y: unset !important;
}

In order to apply that to YouTube, the easiest method is:

(1) Install the Stylus extension:

https://addons.mozilla.org/firefox/addon/styl-us/

If you already have Stylish, you don't need Stylus. If you have Tampermonkey or Violentmonkey, you could try those for step #2.

(2) Then install this user style:

YouTube Left Column Scrollable (March 2018)

Stylus or Stylish: click the blue "Install Syle" button. Tampermonkey or Violentmonkey: scroll down and look for "Install style as userscript".

Screenshot comparison attached.

Note: Stylus and Stylish do not work in automatic private browsing mode. If you have Firefox set to "Never Remember History" you could use a userContent.css file for this fix.