[whpalmer4]

Select the whole block of lines. Edit->Copy. Switch to OmniFocus. Click on the row handle (the dot at the left end) of the row where you want to paste your lines. Edit->Paste. You'll get a row for each line that you copied. If you don't click in the right place, you'll get a single row containing all of the lines, as you have probably already discovered.

I haven't tried this with Thunderbird, as I haven't used it in years, but I believe it will work.

Some other helpful tips:

shift-return will create a new blank row above where you are
option-return will make a new line in the current row
shift-option-return will make a new line above you in the current row

To split existing multi-line rows, select only the text of such a row, then paste as described above. You'll then need to delete the original, and set the dates, context, etc. for the new rows. You can do this more efficiently by selecting them all, then bringing up the inspector to set the data for all selected rows simultaneously.