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Adobe Express doesn't work in Firefox
(lemmy.ca)
When going to use Adobe express on Firefox it comes up with the following message, saying that this browser doesn't play well with others and that I should use Safari, Google Chrome, or Microsoft Edge instead.
Report to webcompat, and try using a user agent switcher like this one to fool the site into thinking you're using chrome and see how it works
It is pretty much impossible to completely spoof a gecko engine for something else.
https://arkenfox.github.io/TZP/tests/engine.html
The vast majority of sites just check the user agent string, so this is not really an issue.