As far as i understand - it depends
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If problem is caused by fontconfig changes - it’s not only Firefox but any program that use fontconfig directly (many people complained about it because everyone use browser therefore start to identify it as Firefox problem, you can search forum to see all those topics)
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If it’s something else that is caused by Firefox only, like engine rendering differences - then it’s only Firefox and your advice is very relevant, it’s good idea to experiment with values