Correctly typed words can receive red underlines when Firefox checks them against a different language dictionary. This is separate from a keyboard-layout problem, which changes the characters produced by the keys. Start with a disposable multiline text field rather than a password or a message ready to send.

Check the active dictionary

Type a short sentence containing familiar words in the language you intend to use. Right-click inside the field and inspect Languages. Select the appropriate installed dictionary. Mozilla notes that not every Firefox locale includes a dictionary, so a missing choice may require adding one through its dictionary catalogue.

Use Add dictionaries from the same menu when needed, review the dictionary’s page and install the intended language variety. Then return to the field and choose it. A dictionary and an interface language pack serve different purposes; changing menu language alone is not the spelling test.

Firefox can check against more than one selected dictionary. Decide whether that helps your bilingual work or hides differences you want to notice. For formal material, keep the intended spelling convention clear to the person reviewing it.

Use a positive and a negative test

Enter one correctly spelled ordinary word and one deliberate typo. Confirm that the first is accepted and the second receives a useful suggestion. Mozilla describes automatic checking primarily for multiline text boxes, so a single-line field is not a reliable control for this test.

If only a specialist product name is underlined, confirm its exact spelling before adding it to the personal dictionary. Added words apply across dictionaries; accepting a typo simply teaches the checker to overlook it later.

Repeat the sentence in the web application where the problem began. The application may supply its own writing tools or handle input differently. Record that distinction if ordinary fields work but one editor does not. Finally, remove the disposable test text. A clean underline pattern is useful evidence, but spelling suggestions do not replace a review of meaning, names and numbers.

Sources: Mozilla Support.