From MozillaZine Knowledge Base
This article describes how to go back to an old version of Thunderbird, including how to deal with problems that may occur due to changes to your profile. Whatever version of Thunderbird you re-install, it will automatically find and reuse your profile (it contains your mail, address books, settings, saved passwords, add-ons etc.), which is not deleted when you uninstall or reinstall Thunderbird. Its also possible to keep the current version, and do a custom installation of a older version in a different directory to see if your problem is due to the latest version of Thunderbird or not.
See this article if you want to go back to an old version of Firefox.
Before you do anything turn off the automatic Software Update feature in Tools -> Options -> Advanced -> Update.
Starting with version 35.1.0 you can not install Thunderbird if you are using XP SP2 (rather than SP3) or are using a version of Vista that does not have the KB2763674 patch due to new signing. The workaround is to install version 38.5.0 and then upgrade. However, that does not let you choose a specific version.  
Go back to a recent release
(3.1.* or later) Except for a backend extension in TB 12.0 (see the note below), all changes in 3.1.* and later versions affect just add-ons and the optional useChrome.css file. This makes it easy to reuse the same profile with an older version.
- Download the version you want from either here or here by clicking on the link for the setup program you want, and then pressing "Save File" in the pop-up.
Mozilla uses win32, mac and Linux-686 directories to store files for Microsoft Windows , Mac OS X, and Linux respectively. The af, be, bg ca .. etc. directories are locale codes. They specify the language plus what country/region any customizations are for. en-US is the USA version of English, en-GB the British version of English, de is German and nl is Dutch for example.For example if you want the US English version of Thunderbird 17.0.8 for Windows chose the 17.0.8 directory, then the win32 directory, the en-US directory, and finally the "Thunderbird Setup 17.0.8.exe" file. Its directory is here. Ignore the "Thunderbird Setup 17.0.8.exe.asc" file (if it exists), that is just a PGP signature that can be used to prove the authenticity of the setup program.
- Uninstalling doesn't delete your profile, but you should back it up first as a precaution using something like MozBackup (Please note: MozBackUp is no longer being maintained, so use with caution.), in case you run into problems.
- Exit Thunderbird and then uninstall it.
- Install the older version in the same location to minimize any side effects. For example, the Windows registry may expect the default email client to be in that location.
- If you have a firewall you might need to reconfigure it for the new executable.
- If Thunderbird disables any of your add-ons you may need to replace them with a version that supports a older version of Thunderbird. Most add-ons at Mozilla Add-ons have a Version Information section at the bottom of the web page. Click on the version you want and it will jump to a download page for it. Lightning for example tends to be very version dependent.
- If you created a optional userChrome.css file you might have to modify it to deal with different window layout/tree structures.
If you couldn't figure out the right locale go to the download page for the fully localized version of the latest version of Thunderbird, hover the mouse over the download link for your locale and see what lang= is set to. That is the locale code.
- Note: Running TB 12 with local mail folders and pop3 mail filters may produce summary files that aren't correctly read by previous versions of Thunderbird. If you decide to go back to a previous version of Thunderbird after running TB 12, you should right-click on each of your local folders and pop3 accounts, then select "Properties" and click the "Repair Folder" button, one by-one for each folder to avoid potential data loss.
From 3.x to 22.214.171.124
Follow the directions in the prior section to download a setup program for 126.96.36.199, but don't install it yet.
- Thunderbird 3 stores the passwords in a different file. The old file isn't deleted when you upgrade so if you haven't changed your password after upgrading it isn't an issue. Otherwise write down your passwords from Tools -> Options -> Security -> Passwords -> Saved Passwords if you can't remember them.
- Exit Thunderbird and back up your profile using something like MozBackup.
- There are several directories/files you may want to delete from your profile
- If Global search/indexing was enabled delete globl-message-db.sqlite
- If you're using OSX delete the .mozmsgs subdirectory. It contains *.mozeml files used by Spotlight Integration
- If you're using Vista or Windows 7 (and Windows Search is enabled) delete the .mozmsg directories. It contains *.wdseml files created by Windows Search Integration
- If you have a IMAP account and Message Synchronizing was enabled you have copies of all of your messages stored as mbox files. Unless you want to use them as offline folders the simplest way to get rid of them is to delete the entire contents of the ..\ImapMail directory. This will also get rid of the *.msf (index) files but Thunderbird will re-create them by downloading the headers again.
- Uninstall Thunderbird.
- Install 188.8.131.52 using the setup program you downlaoded.
- Your toolbar will have changed if you had selected the new toolbar in the Migration Assistant. You can customize it again by right clicking on the toolbar, selecting Customize, drag and drop a button to the toolbar, and then press Done. You may have to do this in several steps if you add multiple buttons, not every version supported dragging and dropping more than one button.
- If you ever selected Unified Folders (originally called Smart Folders) in 3.x you may have a pseudo Unified Folders (or Smart Folders) account listed in the folder pane. This will gradually corrupt your folder listings forcing you to periodically rebuild the index. You can't get rid of it by deleting an account. If you don't have it, you don't have to do anything. Otherwise:
- Exit Thunderbird and back up prefs.js
- Search for Unified folders in prefs.js and find what server id it uses. In this example it's using server4 due to user_pref("mail.server.server4.name", "Unified Folders");
- Find its account id. In this example, it's account4 due to user_pref("mail.account.account4.server", "server4");
- Find mail.accountmanager.accounts and remove that account from it. For example, change user_pref("mail.accountmanager.accounts", "account1,account2,account4"); to user_pref("mail.accountmanager.accounts", "account1,account2");
- Delete all of the settings for the smart folder's server and for the setting that assigns the server to an account. Otherwise Thunderbird will add the smart folders account back. In this example, you'd delete all of the mail.server.server4 settings and user_pref("mail.account.account4.server", "server4");.
- You're done. If you decide to upgrade to 3.x later on make certain you back up your profile first in case you change your mind.