As per the discussion in #3749, the original purpose of the removed condition is unknown, and so we cannot be certain that removing it is safe, but doing so seems the best way of discovering more...
There have been long standing problems with the buttons not working in
Firefox, and now a [further
problem](https://groups.google.com/d/msg/tiddlywiki/GlsruQyPOag/BAhnI2mt
BgAJ) has come to light, and been similarly hard to resolve.
On balance, I’ve decided to remove the undo/redo buttons from the
toolbar (the ctrl/cmd-Z shortcut is still operational), thus avoiding
the problems, and saving some space in the core.
