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
- Type "v" and "V" to get "ü" and "Ü" with tone marks.
- 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.