I want to show/hide the bookmark toolbar in Firefox. The only way to do this at the moment is via Alt + V + T + B. Now, that's a lot of buttons to press for something I'd like to do all the time. Is there a way to automate this process by using any sane combination (two buttons)?
Update: Fixed in Firefox 84 (https://bugzilla.mozilla.org/show_bug.cgi?id=1328637).
The shortcut should be Shift+<modifier>+B on all three platforms now, where <modifier> is either Control or ⌘, depending on the OS.
Now, you open the "Library" with Shift+<modifier>+O (I like the mnemonic "Organizer"), which changed the long-standing shortcut on some platforms and didn't make everyone happy. I for one am thrilled by the change, because I use all three OSes regularly, and having consistency is helpful.
As you know there is no shortcut for this like you would have in Chrome or Opera, and as of this writing (mid-2019) there are no obvious, straightforward ways of customizing Firefox's keyboard shortcuts anymore without doing something like… coding.
There are, as you might be aware, other options like macOS's Keyboard prefpane, AutoHotkey on Windows, and AutoKey on Linux, but I don't know if writing a script just to click a menu item qualifies as "sane" anymore.
The Hide BookmarksBar extension used to be a pretty okay option, and might still be if you are fine with running
one of the ESR releases or (Update: ESR dropped support for "legacy" addons in August 2018) a third-party fork of Firefox that still supports "legacy" extensions.
However, as noted in that other SU post, the "Hide BookmarksBar" extension no longer works in current mainstream versions of Firefox. Given the present limitations of the WebExtension API, it seems unlikely at the moment that the HideBookmarksBar extension could be updated to work again as a WebExtension. This fork of dorando's original keyconfig (which still works with Thunderbird as of this writing) might've also been able to do the job, before mainstream Firefox dropped support for non-WebExtension addons. So, same situation.
For these reasons, I would encourage you to voice your support and/or vote for Firefox bug #1328637 ("Personal toolbar lacks a keyboard shortcut to show/hide the bar") on Mozilla's bug tracker.