[Bug 1711304] [NEW] Extensions with dropdown menu are showing glitches

[Litio]

Public bug reported:

With Firefox recently updated to version 50.0.1 on Ubuntu 16.04.3 LTS
(Xenial Xerus) every extension which comes with a dropdown menu of
options is not shown properly as shown in the screenshot attached.

I tried to purge the Firefox package and remove the .mozilla/ settings
in the user home to provide the conditions to try again and reinstall
the complete package and the result is the same.

It appears to show 2 symbols together instead of only one as down arrow.
When I try to move the extension to the upper space over the address
bar, the situation get worse and one of the symbols is not rendered
correctly and appears stretched.

** Affects: firefox (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: add-on extension firefox

** Attachment added: "Screenshot showing the glitch"
   https://bugs.launchpad.net/bugs/1711304/+attachment/4934094/+files/Firefox.55.0.1.Glitch.png

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Title:
  Extensions with dropdown menu are showing glitches

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