Yes, commonly, I keep the default K-Meleon user string, and it's ok, but it's a bit annoying to have to change and unchange for some site.
As for cookies, or for IETab, I think it would be nice to have this feature.
Mozilla/5.0 (x11; U; Linux x86_64; fr-FR; rv:38.0) Gecko/20100101 Ubuntu/12.04 K-Meleon/76.0
Mail : alain [dot] aupeix [at] wanadoo [dot] fr
Ubuntu 12.04 - Gramps 3.4.9 - Harbour 3.2.0 - Hwgui 2.20-3 - K-Meleon 76.0 rc