You can enable them right now by setting sidebar.verticalTabs = true in about:config
It will appear on the stable channel, when the devs think the feature is "stable"
I know it's probably not what you had in mind but this works very well.
Whoops this was supposed to be a reply to the main post.
You can use Sideberry and userChrome modification until the vertical support is stable and customizable. I'll probably continue to use it even after that because Sideberry has a lot more.
Looks like there are lot of dependencies that are still not resolved. You can watch the current state of bug reports here: https://bugzilla.mozilla.org/show_bug.cgi?id=1894060
References (Depends on 21 open bugs)
Depends on:
1912388, 1912568, 1913510, 1915167, 1916553, 1918041, 1918068, 1919044, 1921951, 1921953, 1921959, 1924193, 1926575, 1927038, 1927422, 1927457, 1896623, 1910576, 1910946, 1911960, 1912391, 1913279, 1913437, 1914438, 1915862, 1916188, 1916941, 1916954, 1917785, 1918044, 1918208, 1918434, 1918726, 1920443, 1920575, 1921190, 1921336, 1922264, 1922307, 1922546, 1923041, 1923052, 1923301, 1923306, 1923363, 1924686, 1925577, 1927230, 1927329
1921081, 1927897, 1928116, 1893655, 1899598
1902032, 1905849, 1893656, 1899336, 1899346, 1899352, 1899599, 1901551, 1905869, 1906888, 1907058, 1909497, 1910203, 1910601, 1918608, 1920691, 1923367, 1924670
Dependency tree / graph
Duplicates:
1698376
1683375
See Also:
1901802
Pls
A place to discuss the news and latest developments on the open-source browser Firefox