It seems to be 'proton' problem Current workaround: 1. Open a tab and enter about:config, click OK after the warning 2. search proton, a few will come up, change all the ones that are true to false. One was already false so I kept it, and the last one had a 3, which I kept. 3. Exit and open FF.
Bug 1713942 Comment 4 Edit History
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It seems to be 'proton' problem - to fix issue with excessive space in menu items. Current workaround: 1. Open a tab and enter about:config, click OK after the warning 2. search proton, a few will come up, change all the ones that are true to false. One was already false so I kept it, and the last one had a 3, which I kept. 3. Exit and open FF.