An update for those wondering if/when a Web Extensions version of Session Manager could be possible:
There are some things that Session Manager does that are still not possible to do with Web Extensions until Mozilla provides the interface (API) to do so. These impact all tab/session restoring extensions, not just Session Manager. Also I can't speak for Morac, even if the APIs are implemented, he may or may not port Session Manager to Web Extensions. I suppose it would be a lot of work.
1. Access, modify, and restore back-forward tab history. This is necessary to be able to restore tabs' histories when restoring a session, in the same way the built-in "show my tabs from last time" does. Otherwise all tab history is lost when restoring a session. That makes it more similar to just opening a set of bookmarks than truly restoring tabs. This is probably the main issue that's blocking Session Manager from working in Firefox 57 the way it does in earlier versions.
- Status: The function/API has been requested, but has not been approved by Mozilla, and nobody is working on it. It was approved for Google Chrome many years ago, and someone was working on it, but it looks like they abandoned it a couple of years ago, and there's no further progress. It seems unlikely to be finished anytime soon.
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https://bugzilla.mozilla.org/show_bug.cgi?id=1378651
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https://bugzilla.mozilla.org/show_bug.cgi?id=1381922
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http://groups.google.com/a/chromium.org ... be7420f4db#
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https://groups.google.com/forum/?fromgr ... 6zY2ZjGSYg
2. Restore sessions without loading all the tabs. Only load the selected tab, and create the others in an unloaded (a.k.a. "discarded") state, until they are clicked on. Important when loading a session with hundreds of tabs!
- Status: The function has been approved by Mozilla for Web Extensions, but at the moment nobody is working on implementing it. However it seems possible to be finished soon, as related work is being done.
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https://bugzilla.mozilla.org/show_bug.cgi?id=1378647
It seems that it would depend on allowing Web Extensions to unload/discard tabs, which is currently being worked on (at least for desktop browsers) and nearly complete, here:
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https://bugzilla.mozilla.org/show_bug.cgi?id=1322485
There may be other issues I don't know about, but these seem to be the main ones that remain. If you want to express your support for Mozilla to implement the necessary functions in Firefox, you can get a Bugzilla account and vote for the above bugs, particularly 1378651. Please follow Bugzilla etiquette, and just vote for the bug without commenting. Comments like "you need to fix this!" aren't helpful.