Very useful tool, thanks a lot.
Only note, perhaps as an add-on to the symbols, tabs can be in different, not so strong colours

[Feature Request]
This extension have so much potential! I mean, it can turn into a tab tag system if you allow others to add various emoji as prefixes with a different hotkeys or a context menu. Think about the tag system in Mac's finder.
Let me explain, for examples, there are different colors of dots symbols in emoji, what if you add a context menu feature/ hotkey to allow users to quickly color tag tabs? Please consider this, that would be amazing!

Why not changing tab background color?

I think it is the only addon that can mark a tab as unread.however as previously said it would be better to make it more easy to be seen.i would suggest that the area of the symbol in the tab to be bigger or maybe the symbol to be replaced with a transparent overlaid image (lets say a transparent single color image).Good job pal.

Good Job.

would it be possible to add a feature to actively flag a tab. I'm not really interested in having unread/unvistited tabs flagged, but only 10-15 from the 500 open tabs, so that I can find these "main" tabs on the fly when scrolling through tabs. Problem is that I need them at that specific place (to find referenced tabs to the left of them) and not somewhere else like most "pin" addons do move pinned tabs somewhere else. Adding such a symbol would just be what I need.

i also wish an option to add the special character at the end or not adding any character. I use this add on to flag the unread tabs and then use userchrome.css to change the unreadtab font color.

Can you add an option to make the character at the end of the title instead of the beginning (as a suffix)?

EDIT (Reply):
Actually, I'm using it with userChrome to apply some styles to the tab, and it doesn't matter whether or not the change is visible (I'm now using an empty space character anyway) as long as the title changed.

This addon is a good start but is not sufficient, because when the user have many tabs opened, a symbol for unread tab is not clear enough to view.
I suggest to add a colored top line (like the active one) for mark the unread tabs.
In this way, together with the code below for unload tabs, is possible have a consistence Quantum style.

/* Colored line for unload tabs */ .tabbrowser-tab[pending] .tab-content { margin-bottom: 2px !important; border-top-style: solid !important; border-width: 2px !important; border-color: red !important; }

best extension for unread tabs, since in Firefox 61 "unread" function was deleted. Thank you very much!

If someone want change color in title from the unread tab, can this make with userChrome.css in profile folder in subfolder chrome with following code:

.tabbrowser-tab[titlechanged] {
color:red !important;
}