Some variations around steps 4 and 5 were required, and eventually Diigo Toolbar defaulted to showing icons without text (not a preference set by me, I wonder how it happened) and I didn't bother with steps 6 through 11 but at least Diigo Toolbar looks normal again. Fingers crossed.
http://www.diigo.com/tools/toolbar shows the toolbar as it should be.
To the best of my knowledge, the loss of icons did not coincide with any update or other significant event.
I have tried disabling, enabling, un-installing, installing Diigo 3.1.6.5.
Any other suggestions?
Postcripts:
* replaced the parts of this message that were lost at time of submission.
* see also http://groups.diigo.com/Diigo_HQ/forum/topic/toolbar-has-disappeared-8686
1. disable DragNDrop Toolbars
- that's the add-on/extension recommended at http://www.diigo.com/annotated/fc687f4ef61f566f5e80302460f3098d
2. quit from Firefox
3. launch Firefox
4. use the Firefox option to customise toolbars
- customisations were previously failing; some misplaced icons could not be removed, etc.
5. realise that the preferred order of toolbars is lost, and can not be changed by Firefox alone
6. enable DragNDrop Toolbars
7. quit from Firefox
8. launch Firefox
9. unlock toolbars
10. drag Diigo toolbar to the required position
11. lock toolbars.
I began working through the eleven-step routine at
http://groups.diigo.com/Diigo_HQ/forum/topic/diigo-3-1-6-5-toolbar-missing-icons-for-sidebar-and-for-diigo-and-bookmark-buttons-menus-8688#2
Some variations around steps 4 and 5 were required, and eventually Diigo Toolbar defaulted to showing icons without text (not a preference set by me, I wonder how it happened) and I didn't bother with steps 6 through 11 but at least Diigo Toolbar looks normal again. Fingers crossed.
right click diigo tool bar
Customize...
Restore Default Set
Good Luck