Firefox's in-built check spelling feature does not work correctly when writing a new message.
A work-around seems to be if you change to another tab, then change back again, the incorrectly spelt word is correctly highlighted.
Is this still an issue for you in the new composer?
As of Riot version v1.5.13 and using Firefox v72.0.2 (64-bit) (Linux) this seems to be working now.
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As of Riot version v1.5.13 and using Firefox v72.0.2 (64-bit) (Linux) this seems to be working now.