When composing an email in mobile view using Firefox Mobile (version 149, though this issue may also occur in older versions), the following problem arises:

It is not possible to enter text into the email text field, regardless of whether the email is being composed in plain text or HTML mode.
The keyboard appears, but no characters are entered. Interestingly, the Enter and Backspace keys still work.
The problem can be resolved by switching to the desktop view - entry is possible there (bug occurs exclusively in the mobile view).
During the initial test in the test system, input worked, but the error occurred after checking/changing/resetting the settings.
Clearing caches and cookies or using incognito mode did not resolve the issue.
Other text fields, such as the search bar in SOGo or forms on other websites, work without issues.
A test in Google Chrome did not reveal any such problems.