Reproduced the issue with Firefox 91.0a1 (2021-06-02) on Ubuntu 18.04. Setting the font to at least 16px or 20px inside Tweaks and playing/stopping a video on youtube or making a move on the Lichess site makes the tab change the height when the Playing message is displayed. I can no longer reproduce the issue with Firefox 98.0a1 (2021-01-20), 97.0b5, and 96.0.2 on Ubuntu 18. Tab height no longer changes when playing and stopping a video on Youtube or making a move inside the Lichess site while having the font set to 20px, 17px, 16px, and 13px.
Bug 1714276 Comment 75 Edit History
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Reproduced the issue with Firefox 91.0a1 (2021-06-02) on Ubuntu 18.04. Setting the font to at least 16px or 20px inside Tweaks and playing/stopping a video on youtube or making a move on the Lichess site makes the tab change the height when the Playing message is displayed. I can no longer reproduce the issue with Firefox 98.0a1 (2021-01-20), 97.0b5, and 96.0.2 on Ubuntu 18. Tab height no longer changes when playing and stopping a video on Youtube or making a move inside the Lichess site while having the font set to 20px, 17px, 16px, and 13px.