When I type this:
This story is a test of the letter "i".
<<set $s="i">>
Maybe <<print $s>>t works now?
qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM
It appears like this:

The "i"s are always misplaced, even if I click "New Story" and test play it without making any changes.
The "i"s are misplaced in Firefox, but not in Google Chrome. However, Firefox displays other people's online Twine games just fine.
I've attached the HTML file that Twine generates with this example.
Comments
Thanks. It's still annoying, but I'll just have to work around it...
Also, for L if he sees this, I get that IE error in Lopsy's above story HTML as well: "Your browser could not save the state of the game . . ."
That make me very suspicious of the computer's integrity.
if it looks perfect in Safe Mode, then one of your Firefox add-ons is interfering with the html file that Twine generated, because Safe Mode is Firefox with all add-ons disabled. To figure out which one, you'll need to disable individual add-ons, restart Firefox if necessary, and then reload your game page. Process of elimination.