Comment 12 for bug 262138

My system has the following:

System:
UBUNTU 8.04 (hardy)
Kernel linux 2.6.24-19-generic
gnome 2.22.3

Internet Browser:
Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.0.5) Gecko/2008121622 Ubuntu/8.04 (hardy) Firefox/3.0.5

Add-Ons:

Extensions:
Firebug 1.2.1
Google Toolbar
Web Developer 1.1.6
FoxClocks 2.5.11
Adblock Plus 1.0
DownThemAll 1.0.4
InFormEnter 0.5.5.4
FoxLingo 2.3.1

Themes:
Default
Saferfox Xpanded 5.0.0

Plugins:
Default Plugin
Demo Print Plugin for unix/linux
DivX(R) Web Player
Helix DNA Plugin: RealPlayer G2 Plug-in Compatible (compatible; Totem)
iTunes Application Detector
Picasa
QuickTime Plug-in 7.2.0
Totem Web Browser Pugin 2.22.1
VLC Multimedia Plugin
VLC Multimedia Plugin (compatible Totem)
Windows Media Player Plug0in 10 (compatible; Totem)

Firefox configuration (about:config) set to default except referer value set to "0"

I tried reinstalling Firefox from nothing and still it lags, however, if I forcefully install the Firefox 3.0 (the original Firefox bundled in Ubuntu Hardy 8.04.1) things go well with Firebug disabled. However, I updated Firefox to the latest available version and my problem goes back.