That really can't be it, at least for me it isn't. I'm logged in with Firefox for Android, so I then try logging on with Vivaldi (Chrome based browser), and I'm able to also log in fine and use both at the same time.
Maybe you just got unlucky with a busy server when trying to log in.
I don't really know. I tried all day and nothing worked; I tried it today again, and still it didn't work. I turned off the ad blocker and nothing. Perhaps is fault of my bad internet?
Seems a very odd situation. I doubt it's your connection, as you're able to log into Lemmy with Chrome.
Have you tried deleting Firefox's cache in Settings>Apps>Firefox? Quit Firefox, then delete its cache, then restart it.
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