While I use an app as my mobile access to Lemmy, I just checked the website with Firefox on my phone and it doesn’t look blurry here.
Do you have specific font settings enabled on your phone? Firefox’s font size will change depending on your settings on Android.
Maybe not entirely the answer you are looking for, but I can at least confirm that it’s not a global issue with Firefox mobile and the website
Ah my bad, I thought you meant on the phone.