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With nothing. Just remove :hover.

i tried it but its not working

So it was working on hover when you used

.tabbrowser-tab:hover { /* some rules here */ }

But it is not working at all times when you relax the requirement with

.tabbrowser-tab { /* some rules here */ }

Do you have any conflicting rules for

.tabbrowser-tab { /* some rules here */ }

or have you tried adding !important to the properties where you need to override the default values?

See also these two older threads:

• /questions/1368296 userChrome.css code
• /questions/1368519 userChrome.css code hide scroll bar