On all SE sites where the left-sidebar exists, this userscript moves the left-sidebar into the topbar, regardless of the SE site-specific preference you've set. Effectively, this acts as if you had checked the site-specific "Hide left navigation" preference on every site in the Stack Exchange network.
Basically, if you don't like the left-sidebar this script makes it so that you don't have to go to your preferences on each site that you visit in order to disable it.
Getting the code
Install from GitHub
Unfortunately, I can no longer recommend Greasemonkey (open source; Firefox only). I try to make sure that my scripts are compatible with Greasemonkey 4.X (GM4), but it lacks features which some of my scripts use, but, for my scripts, I try to make it that their lack just degrades performance. The change to Greasemonkey 4.X was required by Firefox moving to only WebExtensions based add-ons. However, there were significant changes made to Greasemonkey which broke backward compatibility (and there's still a significant lack of functionality). There is no intent to fix backward compatibility. As a result, using Greasemonkey 4.X will result in ongoing issues with older scripts which have not been updated to be used with it. Thus, while I used to strongly recommend Greasemonkey 3.X due to a significantly better security environment (gone in GM4), I recommend against using Greasemonkey 4.X vs. the alternatives.
The source code is on GitHub
Please file any bugs/feature requests as issues there.
You can contact me through Stack Exchange comments or chat.
If there are problems with, or feature requests for, this userscript, please file an issue on GitHub. While GitHub issues are preferred for bugs/feature requests, making sure I get the information (e.g. comments/chat) is sufficient.
This script should be compatible with at least the most recent release of the following browsers: Chrome, Firefox, Firefox ESR, Firefox for Android, Opera, and Edge. It should also be compatible with other browsers.
Left-sidebar in the Topbar's code is released under the MIT License.