I would like to move the hamburger icon to the left, for me it feels more intuitive in that way. Maybe the overflow menu as well.
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While waiting for that to be implemented, it can be moved with CSS.
Put this code into your userChrome.css file:
PanelUI-button {order: -1}
If you're not familiar with using CSS, see How to Create a userChrome.css File.
If you can't get it moved, it would be best to post at https://support.mozilla.org/, where the bulk of people providing support can best help you.
Hello
https://support.mozilla.org/en-US/questions/1447480If you wish to test an code userChrome.css
#PanelUI-button { order: -1 !important; }
Result below
never-displayed