The OpenBSD Firefox package includes the en-GB dictionary. This results in lots of red squiggly lines. There are additional en language packs localized for AU, CA, GB, and ZA, but no US package. There is also a firefox-i18n-en-US package but it doesn’t do anything except print a message that you can change the useragent locale from its default of... en-US.

If you want an Americanized spelling dictionary, you’re out of luck. Debian to the rescue! Start with the hunspell source package page and download the big orig.tar.gz file. Extract it and copy the two en_US files to /usr/local/share/mozilla-dicts/. Restart Firefox and right click in a text area to change language.

Save a step! This is apparently just a bug in Firefox because it should also be looking in /usr/local/lib/firefox-*/dictionaries which includes US dictionary files. These files are a little different (smaller), but they too could simply be copied or symlinked into the above location.