When I open chat window with tabs, each tab have length of contact name. But contacts may have long names (Name Surname Familyname), so in chat window will be to few tabs visible, most of tabs will be hidden and available only via scroll buttons. This is not comfortable to use.
Here is example http://wstaw.org/m/2012/04/28/plasma-desktopBL6141.png - I have 25 tabs open, but see only 4 of them.
But decreasing tab width is not so good idea, because with many tabs we will can't read name of contact and tabs will be not useful, here is result: http://wstaw.org/m/2011/10/24/plasma-desktopQ19118.jpg
This problem is exist already with Kopete, described here: https://bugs.kde.org/show_bug.cgi?id=54194 and this issue got very much votes, so the problem is serious and must be fixed!
Best solution for this problem are:
1. Multirow tabs, examples:
Visual Studio: http://www.c-sharpcorner.com/UploadFile/ReviewImages/11272009103149AM/Tabs%20Studio%20in%20Visual%20Studio%202008.png
2. Vertical aligned horizontal tabs, examples:
Google Chrome: http://www.3dnews.ru/_imgdata/img/2010/07/23/595620/chrome-vertical-tabs.gif
KDEPIM mobile: http://dot.kde.org/sites/dot.kde.org/files/kmail_mobile_maemo1.png
So the problem is serious for usability and must be fixed via any way, or will be best to implement all ways and give ability to select preferred way in options.
First of all, let me address your report itself.
"This problem is exist already with Kopete, described here: https://bugs.kde.org/show_bug.cgi?id=54194 and this issue got very much votes, so the problem is serious and must be fixed!"
That report is from 2003 and has only 295. That's in /no way/ "very much votes". There are bugs with >1000 votes on them and they are way newer than 2003. So this in no way says "the problem is serious".
"So the problem is serious for usability and must be fixed via any way..."
Sorry, are you an usability expert? Where is your usability study to support such claim?
(bunch of provided examples)
Thanks for doing the homework.
==Now to the problem at hand==
I agree that having more than say 5 opened tabs is not really usable with our client, we didn't design it to handle that (btw. did it occur to you that you may be using it wrong? ;) Looking at other very popular chat clients - nobody actually cares about this. I'm not saying we won't, but since nobody ever complained about it, we ourselves give it very low priority as we have more important things to fix (which /are/ serious problems).
Nevertheless, thanks for the report.
Actually I thought it was a very good report, did a good job of stating the problems. and I especially appreciate the research on suggesting several possible solutions. Massive ++ on that.
(Don't put exclamation marks in your reports though, you end up annoying people (see above) :) )
I completely agree, marking as a milestone for 0.5.
I went quickly over the kate's plugin code  (it's in kde:kate/plugins/tabbarextension) and unfortunately it won't be just "take and use" as the code depends heavily on kate's interfaces. So it might actually be quicker to take the idea and write it ourselves.
 - http://quickgit.kde.org/index.php?p=kate.git&a=blob&h=f43df3091eaa05e4dd0be55a23f00f384108cb56&hb=2a5459416bcfabd08d92fe962425c4757fbd9e4c&f=kate%2Fplugins%2Ftabbarextension%2Fplugin_katetabbarextension.cpp
The tabs list button (first screenshot here
http://lh6.ggpht.com/_xvqZvwoH9PI/SzLOE3mIk4I/AAAAAAAAPeE/ba5iQ4VrYGQ/multi_row_tabs.jpg ) is imo the less invasive, multirows and vertical aligned tabs is imho a waste of space, and that's not acceptable on a netbook, so I'd rather avoid that. If you have too many tabs and want to see all of them you can always detach the tab...
tabs list button is good solution, but not solve all problems. I want to see all opened chats with current contacts status (online, offline, away) and alert of unread messages, typing notifications, so all that I see at now in Kopete tabs.
But tab list button often works like menu, so it will show only menu with names and without other info. In this variant it will be not useful.
If we can create tab list button, that shows all other info (status, unread messages, typing notifications), this will be good. But if we do this, will not too hard to implement mode "always show tabs list" that will similar that I describe in this issue.
Daniele E. Domenichelli, and in your screenshot Firefox do at first "Tab list" button (at top) and after that - convert it to multi-row tab bar (at bottom) because it is more useful.
We can do all this features optionally, so for users that don't need this, nothing will broken.
Marking as a duplicate of 308059 because it is basically the same.
*** This bug has been marked as a duplicate of bug 308059 ***