Just wondered how to select text within grid.
Found this http://dhtmlx.com/docs/products/kb/index.shtml?cat=search&q=2186&ssr=yes&s=onselectstart forum entry and started to change my application.
Any way, the solution is too complicated and makes grid unhandy.
Now an idea:
Why not adding a cell type “rotxts”: read-only, text, selectable. Would only require a new css class which disables moz-select. Same for numbers, “rons”.
For now i have to stay with “edtxt” 
Forgot to mention:
“rotxts” and “rons” behave the same as “edtxt” or “edn”.
They make the cells editable.
The difference should be, that the input elements are set to readonly.
That should be defined within the css class too, of course.
I don’t think that creating a new column types is a better solution.
The above link requires only single line of code ( for IE ) and css update for other browsers
if you don’t like to edit original css files - you can copy paste affected rules from dhtmlxgrid.css and redefine them in custom css file, so you will be able to update dhtmlx files and will preserve custom selection logic in the same time.