Started happening ever since I upgraded to Quantum, I think. Idk if it matters but my userChrome.css:
#context-back image,
#context-forward image,
#context-reload image,
#context-stop image,
#context-bookmarkpage image{
display:none !important;
}
#context-back:before,
#context-forward:before,
#context-reload:before,
#context-stop:before,
#context-bookmarkpage:before{
content: attr(aria-label) !important;
-moz-margin-start:31px !important;
}
#context-navigation{
-moz-box-orient: vertical !important;
}
#context-navigation > .menuitem-iconic {
-moz-box-pack: start !important;
}
#PopupAutoCompleteRichResult .autocomplete-richlistbox {max-height:300px!important;height:auto!important}
#PopupAutoCompleteRichResult .autocomplete-richlistitem[actiontype="visiturl"]{display:none!important}
#PopupAutoCompleteRichResult .autocomplete-richlistitem[actiontype="searchengine"]{display:none!important}
#context-sep-sendpagetodevice,
#context-sendpagetodevice { display:none!important; }
#menu_unsortedBookmarks { display: none !important; }
#BMB_unsortedBookmarks { display: none !important; }
#menu_mobileBookmarks { display: none !important; }
#BMB_mobileBookmarks { display: none !important; }