Very promising Rated 5 out of 5 stars

edit: This became a five star add-on with all reasons for being four (and a half:)) star being fixed and improved with version 0.9. USM is the new Stylish: Thanks a lot to the developer:)
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This is a nice addon and a lightweight alternative to Stylish. Not quite there yet, but keeps getting better, very promising. Thank you:)
In the latest version (v.0.8), the chevron of the toolbar icon (the little triangle to display options) is changed and put under the icon -- it was on the side of the icon. Please bring the old look back -chevron to the side- as the chevron adds an unnecessary height to the add-on toolbar (aka status bar) and also looks really bad - icon not being in line with the other icons, etc. Also, the toolbar icon constantly resets its location after firefox restarts.
Thanks again :)

Reply to the developer:
Dear Developer, with your continuous support you are already a 5+ star developer :) the four stars were only for the extension as of v. 0.8, mainly due to the interface issues and as you pointed out aptly, better integration with userstyles.org - I find this add-on really promising and a better future replacement for stylish, as it seems it is aspiring to be as such, based on your comments. As it is, ths add-on is already very useful, and promise 5 stars once this ui issue is fixed (this is not an accessibility issue though, I love to see the addons I like and that have nice icons on the toolbar if I can). For the add-on toolbar issue, however, I think it is more likely to be caused by one of your other excellent add-ons (location bar enhancer), sorry for confusing it here - I made a mention here now just FYI.

This review is for a previous version of the add-on (0.8). 

You can try the version 0.9 for userstyles.org integration and the toolbar button icon fixed. As for the button's position being reset, then it might happen if you have any other restartless add-ons icon as the previous/next sibling to this icon. So you should not keep 2 restartless customizable toolbar buttons side by side as they both rely on their next sibling, and when the first restartless add-on is being enabled upon browser start, the next sibling is not available.