Comment 47 for bug 281348

I have this problem on Karmic clean install with extensions:
Enabled Extensions: [13]

    * Classic Compact Options 1.2.1
    * ColorfulTabs 3.9.4
    * Delicious Bookmarks 2.1.071
    * Download Statusbar 0.9.6.5
    * FireGestures 1.5.4
    * Google Gears 0.5.32.0
    * MR Tech Toolkit 6.0.3.4
    * SubmitToTab 0.3.5
    * Tab Mix Plus 0.3.8.2
    * Test Pilot 0.2.1
    * Tree Style Tab 0.8.2009090201
    * Weave 0.7
    * Web Developer 1.1.8

Disabled Extensions: [1]

    * Ubuntu Firefox Modifications 0.8

If I enable "Ubuntu Firefox Modifications 0.8" and restart firefox, I didn't see the Web developer and Delicious buttons, if I disable and restart, the buttons goes back.