EDIT
Even if Firefox 48+ is technically capable of running WebExtensions, it is deeply wrong to force v3.x on users of Firefox 52 ESR, Basilisk or Waterfox knowingly robbing them many great features in advance.

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Please make the WebExtension version exclusively available for Firefox 57+. Change Geckos minVer for that.
All users of Firefox 52ESR or Firefox forks like Waterfox, Cyberfox, Basilisk etc. do not need the feature reduced WebExtension version to be installed automatically.