In recent update (157) the whole theme of firefox has been changed and I dont really like it, so please change it back to original or leave an option in the settings.
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@porlonsky : for now, you can reverse it with these steps:
URL bar -> about:config -> ENTER -> accept warning -> search nova ->double-click line "browser.nova.enabled" to change to "false"
You can also install the previous theme here:https://addons.mozilla.org/firefox/addon/firefox-proton/
You can also check-out all themes officially made by Mozilla too:https://addons.mozilla.org/firefox/search/?promoted=line&type=statictheme
never-displayed