Really,really sorry about this as I have grown to love fast dial but,since upgrading to FF3.5 RC2 and RC3 I am getting the same "nulljcolor" pop-up as other posters.
Used an Acronis restore to get back to square one and then trace the problem .....and discovered that if I open a cell and then use the back button to get back to the fast dial homepage all is OK but if I open a cell and hit the homepage button then I lose all the cells.
Using the forward button from this blank fast dial screen takes me back to the previously opened cell and the back button brings back the full fast dial page......weird!
It also appears to be screwing my " xmarks" account which comes up with a code 3 error and this cannot be rectified unless I create a new xmarks account.which I have now had to do twice. ....not happy about that!

All in all this is just too much hassle and is quite obviously a conflict with FF 3.5.

I am moving over to "Speed Dial" until such time as you manage to rectify this but will be more than happy to return to fast dial if you can get it fixed.

It is also an issue that fast dial 2.11 always works perfectly but 2.22 (and I posted about this at the time) would not open some of the bookmarks (a completely random selection) in a new tab when accessed from the drop down bookmarks tab in the menu bar.
This does not happen with speed dial.

As I said I am really sorry to be negative but the program obviously requires some tweaking before you get a deluge of the same issues when the majority move over to FF3.5 when it is released in its final form.