In this case, the only thing encrypted with TripleDES is the password itself, so the practicality of a crib or other known plaintext attacks is debatable in my opinion.
If you use the same (or similar) password everywhere, then you have bigger worries than Firefox use of TripleDES. Password stuffing based with leaks from poorly hashed password DB (cough facebook cough) is likely the most practical attack vector in this case.
If all your passwords are like q@qrG#Z4ARYm^qjeTEMN2Kh45v^p7L# then crib like attacks are impractical.
There are other weird/debatable choices in the Firefox encryption layer:
- Why bother with CBC? Things like AES-GCM or other authenticated* encryption mode would be nicer. Not sure it's a flaw here (google the cryptographic doom principle of Moxie Marlinspike)
- Why not wrap the encryption keys with some kind of "key wrap" mode instead. There are such things as AES-KV for instance.
- Why do the weird PBDKF2 derivation here? It's not based on a password the player enters, so there's nothing to "strengthen"? Seems oddly unnecessary (or I don't understand and there's a password somewhere).
- If there's a password then PBKDF2 is really really shit compared to scrypt or even better one the variant of argon OWASP said you should use.