Can I get a combined inbox in Thunderbird?
I believe the feature you are looking for is called 'Unified Folders' which works on both IMAP and POP3 protocols.
The official site location: http://kb.mozillazine.org/Global_Inbox#Unified_Folders
Quote from the site:
Unified Folders (originally named Smart Folders when it was added in 3.0) is a folder pane view which looks like a global inbox account by merging the contents of all inbox folders (both POP, IMAP and local folders) from all accounts. It also shows the inbox of each account as a child folder of the unified Inbox account. Any messages in an inbox shows up in both the root of the unified Inbox, plus the child folder of the unified Inbox for that account.
To use this functionality:
View >> Folders >> Unified
I have verified that this does work on Thunderbird 12.0.1 using Ubuntu 12.04. The KB article I found above it a bit old but still holds correct information about the functionality of the 'Unified Folders'.
If you are just trying to combine the Inbox only and leave the rest as is, it does not look like it can be done but extensions can be made in the future to have this feature in Thunderbird.
Hope this is helpful.
Click view, folders then "unified"
This should sort it for you.
Hope it helps.
I prefer a solution where you have both combined Inbox and Sent folders and the regular folders of each account in your side pane. This is different from the Unified Folders option, where you may end up switching between two different folder views.
Here is how to set up a combined Inbox folder in addition to the standard view, using a saved search folder:
File -> New -> Saved Search
Set Name "All Inboxes" or "Combined Inbox" as you wish
Create as a subfolder of: "Local Folders"
Click "Choose..." and mark each Inbox folder from all your different accounts.
Select "Match all messages"
Create the saved search. You can now find the (saved search) folder "All Inboxes" in your Local Folders.
Repeat the process for "All Sent" if you wish or other types of folders. Of course, you can also set up a saved search folder for all starred messages over all accounts or other useful configurations.
If you add more email accounts later, right-click on the "All inboxes" folder and edit its properties to add the respective folder of a new email account.
Here is more detailed information on saved search folders: https://support.mozilla.org/en-US/kb/using-saved-searches
So I would like to correct ruffEdgz's answer:
If you are just trying to combine the Inbox only and leave the rest as is, it does not look like it can be done
Yes, you can combine your Inboxes using a standard feature of Thunderbird, saved searches, you only need to choose the folders manually and add them when you add new accounts.
I have been using this since on Ubuntu since 11.10 on various Thunderbird versions and also on other OSes. The only thing I haven't tested is POP email accounts, I always use IMAP.