There is a very annoying issue in the Bitwarden Extension for Firefox where characters get lost while performing a search. Seems like somebody from the developers was able to reproduce the issue and proposed a workaround:

TL;DR If you’re on the “Current Tab” view, and attempt to perform a search, especially in a large vault, you can run into this issue.

The current workaround is to ensure that you are on the “My Vault” view when performing a search, as I have not been able to reproduce this issue when starting from there. That’s not ideal, but hopefully this helps until a fix can be made.