Okay, so I just upgraded firefox to FF 3.5 and pretty much immediately noticed two things that bugged me. It seems firefox has decided to take the Internet Explorer approach to tabs: Now, there is a “new tab” button attached to the last tab, and (most annoyingly) when you have only one tab open, there is no close button. Luckily, after a bit of searching, I found the solutions.
Let’s start with problem one: The new tab button.
First I tried the Options: nothing. About:config? Zilch! Thankfully, I found a link on a blog I have since forgotten (if I can find it I’ll post the link here…) to this extention. It works! the new tab button is gone.
Problem two: No close button on last tab.
Okay, for those that don’t understand what I’m talking about (I wouldn’t blame you), when you have only one tab open in firefox 2 and 3, (and you have the tab bar shown at all times) you can close the last tab, and it is replaced by a blank page. This is really convenient when you need a blank page or are trying to close everything without closing the browser. Either way, this was annoying me, but thanks to this post on google groups, I am happy again.
[EDIT: Since then, I''ve found a better solution - check this blog post for more details]
The solution is simple: first go to about:config and make sure that browser.tabs.closeWindowWithLastTab is set to false (typing tab in the search box should bring it up).
Next, we need to edit our userChrome.css file.
If you are using vista, it is located in:
C:\Users\[username]\AppData\Roaming\Mozilla\Firefox\Profiles\[profile]\chrome
In XP:
C:\Documents and Settings\[username]\ Application Data\Mozilla\Firefox\Profiles\Chrome
Thanks to: http://www.makeuseof.com/tag/tweak-your-firefox-with-the-userchromecss-file/
If there isn’t a file named userChrome.css, make a copy of userChrome-example.css and name it userChrome.css
Add the following to your (possibly newly-created) userChrome.css file:
/* Add tab-close-button to last tab*/
.tabbrowser-tabs[closebuttons="alltabs"] > .tabbrowser-tab >
.tab-close-button { display: -moz-box !important; }
.tabbrowser-tabs:not([closebuttons="noclose"]):not
([closebuttons="closeatend"]) >
.tabbrowser-tab[selected="true"] > .tab-close-button { display: -moz-
box !important; }
Save and restart firefox. If all is well, you should have a firefox tab bar that looks (and works) like it used to. I hope they fix this in the next release.

[...] we posted about two problems with firefox 3.5. One of those problems has just become easier to solve.Adding a close button to the last tab in [...]