The problem is this appears to be across all sites.

The zoom setting is site specific. FF will remember the zoom setting for each web site and will revert to normal zoom when a new web site is opened. (New pages on the same web site will retain the selected zoom setting).
Sometimes I do want to zoom the whole page (e.g. for a small picture).
Then you'll want to use the normal Zoom controls of Ctrl+, Ctrl- and Ctrl0 .
posted by x.15a2 Community Leader2 years and 10 days ago Link