Its likely the extension author needs to update the addon, you may wish to notify the developer by using contact info on addons.mozilla.org

You could try posting in the All In One Sidebar support forum.

Before doing that, you may want to troubleshoot all your extensions to make sure that something else you have installed isn't causing or contributing to that fault.
http://support.mozilla.com/en-US/kb/Troubleshooting+extensions+and+themes

I've asked to All-In-One-Sidebar too. I've posted here because Firefox updating seems to be no stranger to modify windows theme. I recently had to install "Theme Font & Size Changer" to recover a very tiny default font size after an update.

I don't use the All-In-One-Sidebar, but my Bookmarks side bar is also double spaced. I have tried in Safe Mode (no add-ons), an out of the box new profile, in a virtual machine and on two other machines. All have the updated Firefox 25 versions and all have this annoying issue.

Shelby13,

Are you using Win7 as Yamato96 is?

No - sorry, perhaps I should have mentioned that Windows 8.1 is being used.

There's another thread about this issue here - https://support.mozilla.org/en-US/questions/975714

Thanks - have added comment there.

I have this problem as well, and it's not my sidebar extension. I tested using a new profile, with zero extensions, and the problem occurs on the new profile too.

Also, a very, VERY annoying problem happens, whereby when you hover over your bookmarks in the sidebar or the bookmark manager, and move your mouse downward, white lines appear between the bookmarks, and when you move your mouse up, the lines disappear.

What the heck is going on?

colaclone,

Did you read this support thread?
https://support.mozilla.org/en-US/questions/975714