Do you mean that the page content is magnified or is this about the Tab bar (user interface area) ?

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

here you go

That is the full screen

Is only that last tab affected regardless whether it is selected or not when you close the lid ?

Did you try to reboot the computer in the meantime ?

The page isn't zoomed (i.e. resetting the page zoom doesn't have effect) ?

• https://support.mozilla.org/en-US/kb/font-size-and-zoom-increase-size-of-web-pages

I 'refreshed' Firefox, I rebooted my computer, no change. This just started happening. I leave my gmail tabs open. When I close the lid of my PC laptop then reopen the tab on the right will be magnified and the last email opened, weird. Refreshing doesn't reset it. I must close the tab and reopen.