Known issues with the Firefox Pinyin tone mark input extension include:
- Faulty conversion when the tone mark falls on an initial vowel.
- E.g. Oū, instead of Ōu; An instead of Àn; Aǐ instead of Ǎi
- Inability to add tone marks to ü and Ü
- The tool will occasionally remove numerals from year dates. A pattern has not been determined for this, but it seems to predominantly affect the digit 1.