I don't have that on fenec and ironfox on my pixel 6a running graphene.

What I have always had with any firefox based browser on android is erratic behavior in text field. Most of the time it works well but sometimes on some commenting systems my input is duplicated/multiplicated/garbled, trying to select where you want to insert words in the middle of the sentence sometimes becomes impossible, always resetting to the beginning of the text field, etc.

On some websites it only rarely happens, on some it is much more regular. Never understod why but when I want to edit a comment I have to resort on a regular basis to copy the full comment to a note app, edit the text, and replace it in its entirety in the text field.

Super annoying but still less annoying than using a chrome based browser with no way to remove ads and have a bit of privacy control.