Sidebery
Sidebery is not a dependency of FF Ultima and is completely optional, while configuration does exist (below), it is not the primary focus of the theme.
Here's a preview of Vertical Tabs & Sidebery side by side. Can you spot the difference?
Well yeah, you can spot the difference. But the point is.. Sidebery is automatically themed to be as consistent as possible to the native vertical tabs. This theming is only based on the colors of the current Color Scheme, so configuring your sidebery in other ways is still possible (since nothing else is modified).
Sidebery Only Setups
The following settings help create a "Sidebery Only" setup
ultima.tabs.hide-for-sidebery- will automatically hide the tabs bar (vertical or horizontal) whenever Sidebery is open.
Sidebery Autohiding
By default Sidebery Autohide collapses the Sidebar into a ~40px width. Collapsed width can be altered by combining the following settings:
ultima.sidebery.autohide+ultima.sidebery.autohide.4pxultima.sidebery.autohide+ultima.sidebery.autohide.120px
Sidebery Extension Settings
This is for best compatibility with the theme, other configurations should work just fine but they are unaccounted for.
Basically a top down format works best with FF Ultima's themed Sidebery. Pinned tabs on top, and Navigation horizontal or hidden.
Again, you don't have to use these extension settings, and you can even disable Ultimas Sidebery theming with ultima.extension.theme.sidebery
Personal Configurations
If you ever want to change the Sidebery theme, either change your Color Scheme ultima.theme or turn off ultima.extension.theme.sidebery to use default Sidebery (unmodified). You may be able to modify FF-Ultima's configuration in Sidebery's Style Editor too (untested).
If you scroll down to Other in the Sidebery Style Editor. FF Ultima uc variables are recognized and editable. I haven't tested these much but it was surprising to see that Sidebery can detect them.