https://support.mozilla.org/en-US/questions/963759 If you want to set the text size/pixel density in firefox different from that of your system’s settings like it was handled in prior versions, enter about:config into the firefox address bar (confirm the info message in case it shows up) & search for the preference named layout.css.devPixelsPerPx. double-click it and change its value … Read moreHow to change Firefox’s default zoom level
Type about:support in your Firefox address bar. Just click the Show Folder button to go straight to the profile folder for that Firefox installation. Backing up your profile is as simple as copying that folder (Make sure firefox is closed). For Windows users, you can also try MozBackup to backup and restore.