There are so many Balmora expansion mods, I don't know which one that is. Anyway, I don't think I've ever used one. With the graphics mods I'm using, along with Starfire's NPC Additions and Children of Morrowind my FPS is already low enough in Balmora.
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Its this one - the mother of all Balmora expansions:
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=373
Plus Slartibartfast has a texture fix to it as well:
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=4324
Here is the Lite addition by Slartibartfast to help with fps:
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=4382
And I havent even looked into the recent update yet:
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=8647
I wish I knew how to make these links into shortened phrases, never learned how to do it sorry....