The loss of this plugin kept me at Firefox 4 for about 1.5 months now.
I live and work in the cloud and cannot be without my beloved Elasticfox*.
Today, I have enabled this "incompatible" add-on in Firefox5 using this method.
Open Firefox and open a new tab.
type in about:config
Read and heed the warning!
In the Filter: box type in extensions.checkCompatibility and press <enter>
If nothing pops up, right mouse click on a blank area in the result and select New
Then Boolean
Then type in "extensions.checkCompatibility.5.0" (without the quotes) and set the Value to False.
Restart Firefox.
I suspect that Firefox 6/7/8/* can be manipulated the same way but with an appropriate string modification of extensions.checkCompatibility.[678].0
The great UNDO should things go south:
in About:config > Filter "extensions.checkCompatibility.5.0" and right mouse click and select Reset and restart Firefox.
Enjoy.