I had no problem with https://www.youtube.com/watch?v=_TshrvMfxoQ

https://www.youtube.com/watch?v=_TshrvMfxoQ&app=desktop took me to the same webpage.

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

If there is still a problem, Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Did this help?

While you are in safe mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}

The "m" is the mobile layout, and I suspect YouTube (or possibly an add-on) is redirecting from desktop to the mobile site. Firefox itself doesn't have a feature to do that.

Could you test in a private window (Ctrl+Shift+p or right-click the www link and Open Link in New Private Window)? That would bypass cookies saved by YouTube in Firefox that are used in regular windows. If it works normally, then try logging in to YouTube in the private window and see whether it still works normally. If so, I suggest deleting your YouTube cookies in the regular window. Here's how:

While viewing a page on the site, try either:

• right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info > Security > "Clear Cookies and Site Data"
• (menu bar) Tools > Page Info > Security > "Clear Cookies and Site Data"
• click the padlock or "i" icon in the address bar, then the ">" button, then More Information, and finally the "Clear Cookies and Site Data" button

In the dialog that opens, you will see one or more matches to the current address so you can remove the site's cookies individually without affecting other sites.

Then try reloading the page. Does that help?

First, thanks for the very quit replies. Cheers to FF Support for that.

Yes, thanks, I am aware that m is the mobile layout. So odd that it began showing up in the desktop version of the browser.

Before trying any of the above I decided - log out of the google session - log out of FF Sync not merely log out of my browsing session - and I closed FF down.

Then reversed the steps.

Bizarrely all is well now.

^ quick not quit (sorry)

As it seems the problem has gone away, "on its own" I shall mark this as Solved. Cheers.

Good to hear. Safe surfing.