That happens if the Option key is pressed if you start Firefox.

See Safe mode

Thanks for your answer, much appreciated and I now understand why. But I use the Option key as part of my QuicKeys string to load an application without needing to mouse around. When you've used a pc for over twenty years, your arms get so tired using a mouse that mousing is something to avoid if at all possible...

So it would be nicer if the panel that appears had a non-greyed out option to ignore the safe mode and load Firefox normally. I can only do that currently if I mouse to check one of the displayed options (that I don't actually want)...