Setting max tabs count doesn't work with Firefox 52

[v6ak]

I looked for replacement of the old addon before I get updated Firefox ESR to newer version. With Firefox 52, it works, except I cannot change maximum number of tabs, because the Save button does nothing.

Maybe it is OK to ignore the old Firefox (the old extensions works well there), but in such case, it would be better to mark the extension as incompatible or to mention it as a known limitation.