A dark purplish theme for Firefox
Right now there is now way to change the hover background color of a tab. Though, if you still want to change this value you can add the following lines in userChrome.css which should be located in your profile directory under chrome directory (<YOUR_PROFILE_DIRECTORY>/chrome/userChrome.css).
toolbar:-moz-lwtheme-brighttext .tabbrowser-tab:hover>.tab-stack>.tab-background:not([selected=true]) {
background-color: var(--toolbar-bgcolor) !important;
}