It's definitely not Adblock Plus, as I checked that. Below is a list of installed extensions, themes and plug-ins (as produced by Extension List Dumper):
Application: Firefox 3.6.3 (20100423154712)
Operating System: Linux (x86_64-gcc3)
EXTENSIONS:
- Adblock Plus 1.2
- Add to Search Bar 2.0
- British English Dictionary 1.19
- ColorfulTabs 4.4
- Extension List Dumper 1.14.8
- Favicon Picker 3 0.5
- Places’ Full Titles 3
- User Agent Switcher 0.7.2
- Verificação ortográfica pt-BR 1.4
THEMES:
- animated firefox 1268221304
- Dark Fox
- Default 3.6.3
- Firefox B
- firefox_blue
- Fire Fox Grau Blau
- Firefox Pavoninus
PLUGINS:
- DivX Browser Plug-In
- Java(TM) Plug-in 1.6.0_20
- mplayerplug-in is now gecko-mediaplayer 0.9.9.2
- QuickTime Plug-in 7.6.4
- RealPlayer 9
- Shockwave Flash
- VLC Multimedia Plug-in
- Windows Media Player Plug-in
Here are some screenshots from, in order, Firefox in Windows 7 (same laptop), Firefox in Vista (different PC), Firefox in Sabayon Linux (same laptop) and Konqueror in Sabayon Linux (same laptop):




I don't know whether you guys can see a difference, but it's very noticeable at my end: Firefox in SL is really ugly. Can you see that even the smiley looks terrible in Firefox under SL? It must be a GTK-related thing, because SL Forums in Konqueror looks fine. By the way, the vertical lines I complained about in my first post disappeared -- I don't know whether that was after a reboot or after tinkering with themes, fonts etc. a few days ago. But the fonts are still awful.