Install Firefox 3.0.19 on Ubuntu 8.04

Hello

I would install the newest Firefox release but it failed. I've still the old version 3.0.19. And I have a problem. If I will call a web site, I get the following alert and the site will not appear.:

ASSERT: *** Search: _installLocation: engine has no file!
Stack Trace:
0:ENSURE_WARN(false,_installLocation: engine has no file!,2147500037)
1:()
2:()
3:()
4:epsGetAttr([object Object],alias)
5:()
6:SRCH_SVC_getEngineByAlias(chatmani.ch)
7:getEngineByAlias(chatmani.ch)
8:getShortcutOrURI(chatmani.ch,[object Object])
9:canonizeUrl([object KeyboardEvent],[object Object])
10:handleURLBarCommand([object KeyboardEvent])
11:anonymous(textentered,[object KeyboardEvent])
12:fireEvent(textentered,[object KeyboardEvent])
13:onTextEntered()
14:handleEnter(false)
15:onKeyPress([object KeyboardEvent])
16:onxblkeypress([object KeyboardEvent])

What fo I have to do, that Firefox runs correctly? Thanks a lot.
Peter