Open the 'Config Editor' via the Tools-Options-Advanced-General menu. Smile at the 'There be dragons' warning and continue. The 'Config Editor' presents a long list of keywords next to their assigned values, similar to the 'Registry Editor' in Windows (hence the warning). Type
quicksearchin the filter text box, which will return the preference
Double-click the quicksearchquery entry to edit it. The field is a complicated logical formula of the format
with each (..) item being something like
The important point to note here is the list ends with double brackets i.e. )) which makes sense if you look a the overall structure.
Your job is to insert another entry in the (..) list:
In simple terms this means putting that entry between the double brackets at the end of the list, so you end up with something like:
If you insert the new value as (..) and don't have still have a double bracket at the end of the list, then you've messed up.
I'm not sure when Thunderbird reloads the config values you've just editted, but in my case it seems to take an exit and restart of Thunderbird to get the settings to take effect...
Now when you use the search in the address book, it will look for your string in the Notes field as well.
To find the names of the othe fields if you want to include others in the search, type
attrin the filter text box, and you will find the various field names.