There are a few things you can do to turn on spell check in Firefox.
First, you can try opening up the Firefox settings. To do this, click on the three lines in the top-right corner of the Firefox window, and then click on "Options."
From here, click on the "Advanced" tab, and then click on the "General" tab. From here, you should see a setting that says "Check my spelling as I type." If this option is checked, then spell check should be turned on in Firefox.
If this option is unchecked, you can try checking it and seeing if that turns on spell check. If it doesn't, you can try restarting Firefox and seeing if that makes a difference.
If you're still having trouble, you can try this link: https://support.mozilla.org/en-US/kb/dictionary-download-and-installation-firefox?redirectlocale=en-US&redirectslug=dictionary-download-and-installation
This link will take you to a page on the Mozilla website that has more information on how to download and install the correct dictionary for spell check in Firefox.