Email programs sometimes do funny things to email addresses. In Thunderbird by default, outgoing email addresses are added to a ‘collected addresses’ address book, and then other addresses that you say you want to add to your address book are added to Personal Address Book (unless you’ve changed it). This leads to address chaos.
Amalgamating address books / contact lists is definitely something good to do.
It’s actually very simple to do in Thunderbird, amalgamating or merging address books.
Open Thunderbird -> open address book -> if it’s not already there, make sure you’ve got directory pane opened from View -> select the address book you wish to move -> select the contacts you wish to move (ctrl+a works for all) -> drag them to the address book you wish to add them to.
All the better if the address book you’re merging into is synchronized somewhere.
Don’t forget to also check out the default settings so that each of your email accounts saves email addresses where you want it (for me, this is one single address book that’s synchronized between all of my devices).
Hope this helps!