For lower Firefox versions:
If you find that your context menu is still dark, you may need to update your Firefox or fix it manually by following the steps below:
Follow the guide at
https://www.userchrome.org/how-create-userchrome-css.html to add a userChrome.css for your Firefox.
Then, copy the code below to your userChrome.css.
menupopup {
--menu-color : #15141a !important;
--menu-background-color : #fff !important;
--menu-border-color : #eee !important;
--menuitem-hover-background-color : #eee !important;
--menuitem-disabled-hover-background-color: transparent !important;
--menu-disabled-color : #aaa !important;
--panel-padding : 4px !important;
}
Finally, restart your Firefox.
