Using webmail with your email client
From MozillaZine Knowledge Base
Thunderbird supports using POP and IMAP mail servers for fetching/reading new messages. It does not support webmail. If you want to use webmail (or WebDAV or HTTPMail) with Thunderbird you normally need to use an add-on that emulates a POP server. It has to do some type of web scraping (an automated way to read the contents of the screen) to do that, which means it may break if the layout of the web page changes. If that happens you can't use it until an updated version is available. The webmail extension (the most popular add-on that does this) hasn't been updated since 2014.
The best solution is to use a real mail server unless you are in an environment where an admin has configured a firewall to prevent you from using a mail server on the Internet The following email providers provide a free POP, IMAP and POP server with all free webmail accounts
If you check "Tools -> Account Settings -> Server Settings -> Leave messages on server" you can access your mail both from Thunderbird and from a browser. You may want to set the number of days it keeps a copy to avoid filling up your webmail mailbox.
The WebMail extension for Thunderbird enables access to Yahoo, Hotmail, MailDotCom, Gmail, AOL and Libero webmail using Thunderbird. It has a dedicated thread in the extension/theme releases forums and a Thunderbird Webmail Extension Google Group.
It is useful when a firewall blocks access to a POP or SMTP server. For example, if your firewall only allows connection to port 110 for POP and port 25 for SMTP you can't connect to Gmail's mail servers. But you can still use Gmail using the WebMail extension since it uses port 80 with http to send and receive messages. The FreePOPs add-on could also be used to workaround the firewall but it doesn't support sending messages.
Sometimes the webmail extension breaks when the format of the webmail pages change. If you don't want to wait for a new version to be released, you can usually get beta versions from the Google Group. While it's rare for a beta to do any harm, it's a good idea to back up your profile before trying one.
All of the released versions are available at http://downloads.mozdev.org/webmail/ . That is useful if you are using Thunderbird 3.1.12 because while the email provider specific add-ons seem to support Thunderbird 3.1 and later, the latest version of the webmail add-on usually requires recent versions of Thunderbird.
Accessing other folders
Normally you can only fetch new messages from the inbox folder because the add-on (or webmail extension) emulates a POP3 server. However some add-ons let you specify that the contents of other folders should be transparently merged with the inbox folder when checking for new mail. For example, if you're running Ypops! to use Yahoo webmail right click on the Ypops! icon in the system tray, choose "configure", and then "download folders" to configure what folders get merged. This is completely transparent to Thunderbird, which thinks it's fetching new messages from the inbox folder on a POP3 server.
If your add-on doesn't support this check whether your webmail GUI has an option to disable the bulk mail or junk mail folder. That would let you filter Spam messages using Thunderbird (if you want to).
Exporting Yahoo Mail Messages
If you are using Yahoo webmail another solution would be to switch to a email provider that provides free POP, IMAP and SMTP servers such as Gmail, and migrate your mail messages to Thunderbird. Yahoo webmail doesn't support exporting messages or folders. However, this thread provides instructions on a way to export the messages and then import them into Thunderbird using several tools. The basic steps are:
You can migrate your contacts by exporting them as a .CSV file using Addresses -> Import/Export -> "Yahoo! CSV" and then importing that file using "Tools -> Import -> Address Books -> Text Files.
If you get a "connection to server pop.mail.yahoo.com timed out" error message, it's because you didn't specify localhost as the mail server when configuring either Ypops! or the webmail extension. There would be no reason to use those add-ons/extension if you had the right to access the email providers POP3 server. Instead, you're using a local proxy (which you specify as either localhost or 127.0.0.1) that knows how to access the webmail account while emulating a POP3 server for Thunderbird.
If you have problems connecting, check that you have a single copy of the add-on running. You might not have configured the add-on to run automatically when you boot. If you accidentally run two copies of Ypops!, it won't connect, but only one copy will show in the system tray. You can see the second copy using Windows Task Manager (Ctrl+Shift+Esc).
Double check you didn't enable a secure connection in Tools -> account settings -> server settings unless the add-on's instructions explicitly say it needs SSL or TSL. Experiment with setting the absolute minimum number of settings/features. For example the Ypops! v0.84 release notes state to not check "plain text" and "secure transmission using MD5" in its advanced preferences -> security settings. You'd normally assume it's safe to use those features since they're part of the add-on.
If the instructions suggest using non-standard ports such as 111 for POP to avoid interoperability problems with your anti-virus program, do what they suggest. But if it doesn't work, temporarily disable your anti-virus program and use the standard port . If that works, then use whatever port the instructions for your anti-virus program say to use, re-enable your anti-virus program, and see if it still works. For example many email providers don't support port 111, and Ypops! works fine with Avast! using port 110. It's a good idea to disable any anti-virus checking on outgoing messages as it can cause interoperability problems, and if the recipient doesn't have an anti-virus program, they have bigger problems to worry about.
If you're using the webmail extension and get the error message "Sending of username did not succeed. Mailserver localhost responded: undefined is an unsupported domain", it's because it requires you to enter the domain name as part of the username.
You may need to upgrade to a later version of the add-on or the webmail extension if your provider changes the layout of the webmail site.
If none of this helps, browse the issues articles for Thunderbird .