Is it ONLY Gmail or is everything sized up? I ask that because... Firefox 103 now applies the Windows "Make text bigger" setting as a '''zoom multiplier''' ([https://www.mozilla.org/firefox/103.0/releasenotes/ Release Notes]). Your options: (A) If you do not need "Make text bigger" for other applications, the simplest thing you can do is reset that to 100% to resolve the issue: <blockquote><img src="https://user-media-prod-cdn.itsre-sumo.mozilla.net/uploads/images/2022-07-27-08-29-38-5663aa.png"></blockquote> ---- (B) Firefox 103 has 3 interrelated hidden settings that control the overall zoom level and whether it considers information from the system. This one seems to be working for many people to revert the change: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button accepting the risk. ''More info on about:config:'' [[Configuration Editor for Firefox]]. ''The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.'' (2) In the search box in the page, type or paste '''ui.textScaleFactor''' and pause while the list is filtered. Normally, Firefox will display a bar with Boolean Number String and a plus button. ''If Firefox displays a row that already has a number value, skip to step 5.'' (3) Click the circular button in front of Number and then click the + button to create a new preference. (See composite screenshot below.) (4) Firefox should display an editing field, and show a value of '''0''' -- edit the value to '''100''' then press Enter or click the blue check mark button to save the change. (5) If the current value of '''ui.textScaleFactor''' is NOT 100, then double-click the preference to display an editing field, and change the value to '''100''' then press Enter or click the blue check mark button to save the change. <blockquote><img src="https://user-media-prod-cdn.itsre-sumo.mozilla.net/uploads/images/2022-07-27-08-13-05-8984a9.png"></blockquote>

Gmail has changed its interface. Yesterday it switched me to it without my wanting it. If that's the issue, you can switch back to the old interface.