Currently, Firefox doesn't allow tabs to be individually colored in the tab bar. The ColorFulTabs extension creates a vertical tab sidebar with colored tabs. It also has a toolbar button that displays colored tabs in a popup when clicked, though not many tabs are shown at once. The extension has the 'Access your data for all websites' permission which means that it could be updated to inject code into any websites to steal passwords or such. The chances of this happening are probably exceptionally remote. If you are worried, then you could install the Tree Style Tabs extension which is manually reviewed by Mozilla for safety as well the TST Colored Tabs extension which has no unsafe permissions. That would duplicate the behavior of ColorFulTabs. Another solution would be to install Firefox Nightly alongside Firefox Release (or beta). This would allow you to run two Firefox programs at the same time. You can run Firefox Release and only install recommended extensions (vetted by Mozilla) and use it for sites you want very secure like email, banking, shopping, etc. In Firefox Nightly, you could install extensions like ColorFulTabs that aren't monitored by Mozilla and use if for the rest of your web browsing where it wouldn't hurt much in the unlikely chance you had any passwords stolen by a rogue extension.