Idea 1
With the majority of the world doing top-posted emails; giving Thunderbird the ability to flip the email so bottom is top and top is bottom. This would be fantastic! No need to scroll down to the bottom of an email and read up then down then up again. Basically flipping in at a moments notice.

Idea 1A
This is an extended idea.
1) Default view “bottom posted” flipping the email. Always display in a bottom posted manner.
2) Being able to send email in top posted fashion for all the Outlook users but viewing the email in bottom posted style.

One step in stopping the MS insanity or minimizing its pain on the world.

Another cool feature would be a simplified HTML ot Text-Preview of an attached spreadsheet direct in Thunderbird.

2) With respect to Mail merge here are a few possibilities:

a) With respect to contacts in the address book. Being able to send multiple personalized emails via the address book. The functionality should allow sorting by Company, Department and title. In essence sending out multiple, personalized emails via some enhanced database functionality in the address book. Being able to add additional fields/columns to the address book would also be nice touch. Some companies use different titles for the same position.

b) Updating and personalizing document in OO.o via the address book in Thunderbird.

c) Combining both features: Updating letters in OO.o and personalize emails via Thunderbirds address book.

4) Right clicking on an attachment and selecting “save as” and saving it as a different file type. Thunderbird calls oo.o and does a “save as” conversion. Doc to odf, pdf to text/odf/doc.

PS: I am unsure of the level of support that pdf will have in oo.o. I have heard that editing pdf’s will be an option.

Just a thought.
Ok. I am done. Really.

Just a thought; reader and or a previewer of an attachment. A previewer of the most common attachments.

* Some sort of mail merge feature?

1) Saving a document; also having it send it to an email recipient/group.

2) This would apply to oo.o: tracking the usage of a document and as the document changes it would alter the user to resend it to a previous list of recipients. Basically associating document names to a list of people in an address book.

3) Expand search to query inside certain (attached) document types; using oo.o as a converter. Caching the data for quicker look ups? Of course the ability to purge the index and search query at the users discretion.

3a) Having certain words not indexed, IE common words: and, the, a, etc.
3b) Specifying words/files to be indexed. Creating a list of keywords that should be indexed.

Use what you like; discard the rest.

As about other “stoppers”, do not hesitate to ask in mozilla.support.thunderbird or forums and may find solutions quicker than expected.
[exchange is another discussion, but calendar is there, rss too, weather in statusbar etc as extensions or native and improved by extensions ..]