Some extensions that have preferences built into the addon page utilize their own css files and don't use all the Firefox default styling settings.
As such, compatibility issues or ugly appearances may arise.
Please see the update here.
If you run across any addons with this issue, please list them here and I will get out a fix when I can.
In addition, please mention the addon as either (1) compatibility - text unreadable or (2) ugly design.
As always, if you are willing/able to help create a fix for an addon, please submit a pull request or post the code somewhere and link to it here. Not much goes into each fix, but I am only one person and we don't all use the same extensions. See here for addons that have currently have fixes and for a general idea on how to create a fix yourself.