First I assumed this was a one-off and went into "tools", "options", "general" tab, "home page" and reset it back to my usual preference "about:blank".
The next time I restarted Firefox it had reverted to the u-search entry again.
I've spent the afternoon scouring the net for a solution and was advised, amongst other things, to check various entries in "about:config". Sure enough there were seven different u-search entries (listed below), which I modified back to default settings. Yet the config file entries all reverted back to u-search on restart.
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
Do not click the Reset button on the Safe mode start window or otherwise make changes.
Thanks very much. I did exactly as prescribed and yes, no sign of u-search.
I do still believe there would be benefit in optionally allowing users to accept or reject changes to the config file(s).
I also believe your prescribed suggestions should be presented prior to other problem users entering their own questions (e.g. this support site already recommended some checks before I asked my question - latest update and Firefox reset could potentially be added to that list, perhaps?)