This will probably need splitting off into individual bugs. I think not scaling favicons with text size is intentional - they are not text, and you're not using full zoom (DPI / window.devicePixelRatio) scaling, you're just changing minimum font sizes. That combined with the fact they'll look ugly when scaled by a factor of, say, 1.16, and given that we don't know what counts as the "base" or "normal" font size on Linux, means I'm not sure we can do much without ruining their appearance there completely. The URL bar, on the other hand, probably should scale vertically when text size increases. I'll morph this for the URL bar. If you think my judgment of the favicon issue is wrong, I'd suggest filing a separate bug in Firefox :: Tabbed Browser with more details.
Bug 1753582 Comment 2 Edit History
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This will probably need splitting off into individual bugs. I think not scaling favicons with text size is intentional - they are not text, and you're not using full zoom (DPI / window.devicePixelRatio) scaling, you're just changing minimum font sizes. That combined with the fact they'll look ugly when scaled by a factor of, say, 1.16, and given that we don't know what counts as the "base" or "normal" font size on Linux, means I'm not sure we can do much without ruining their appearance there completely. The URL bar, on the other hand, probably should scale vertically when text size increases. I'll morph this for the URL bar. If you think my judgment of the favicon issue is wrong, I'd suggest filing a separate bug in Firefox :: Tabbed Browser with more details. Ditto if there are other components where you're noticing issues.