I lost my spell check with the last update

I don't think Firefox normally has a spell-check icon. Was it on an add-on toolbar? Do you recall Firefox saying that particular add-ons would be disabled for compatibility reasons?

Let me guess that you used the spell check on the Google toolbar. Extensions for Firefox, such as the Google Toolbar, include a list of compatible Firefox versions. Currently, the Google Toolbar only goes up to Firefox 4. However, people have tested and it actually works on Firefox 5, so there are two workarounds:

(1) Edit a file to revise the range of compatible versions

(2) Install an add-on that lets you ignore the range of compatible versions

This thread has info on both approaches: google toolbar does not work with firefox 5.0. why not! | Firefox Support Forum | Firefox Help.

If it wasn't the Google Toolbar, a similar strategy still might work...

I really do not understand this. Can you please tell me, in super simple terms how to fix this?

The problem from my end is I don't know where your spell check icon was coming from, so I can only give you my guess that it was the Google toolbar. If you check here, is the Google toolbar disabled?

orange Firefox button or Tools menu > Add-ons

On the left side, click Extensions, and check the grayed out ones.

Firefox doesn't have such a spell check button if you mean a button that appeared on a toolbar at the top of the window.
In that case the button has been provided by an extension (Tools > Add-ons > Extensions)

Do you see any disabled extensions on the Tools > Add-ons > Extensions (about:addons) page?

Make sure that you have a dictionary installed and selected.

See also:

• https://support.mozilla.com/kb/Using+the+spell+checker

• http://kb.mozillazine.org/Spell_checking
• http://kb.mozillazine.org/Dictionaries