Sebastian recommended the HTML Temp add-on for the Thunderbird email client to switch HTML messages in Thunderbird on or off more effortlessly.

Download HTML Temp from the official Thunderbird add-ons repository. Once it has been downloaded open up the email client and click on Tools > Add-ons to open the add-ons manager. Click on the options icon on the left of the search all add-ons form there and select Install Add-on from file from the context menu.

Select the downloaded add-on on your local computer and install it in the email program. You need to restart Thunderbird before it becomes available.

The add-on adds the current message display preference in the lower right corner of the status bar. You can now right-click on it to switch to Simple HTML or Original HTML whenever you need to view a message in HTML. Once you are done you use the same method to switch back to plain text messages.

The add-on allows you to move a button with the same functionality to one of the toolbars. This can be helpful if you have disabled the status bar in the program or prefer to click on the button in the header area of the email client.

Update: Sebastian mentioned that the toolbar button behaves different from the status bar button. When you click the toolbar button HTML will be enabled only for the active email and no other. This means that you do not have to click again to restore the plain text message view.

Use BCC Instead is a Mozilla Thunderbird extension. It works almost like the native attachment warnings that you get when you use words like attached in the email body or subject without attaching a file to the email.

The extension warns you whenever you send emails to multiple recipients using either To or CC instead of BCC.

Please note that the extension warns you about sending the email to multiple To or CC recipients. It gives you the option to go ahead and send it anyway or to click cancel to modify the parameters to BCC. It does not auto-correct the issue.

The default limit is set to 10, which means that you only get warnings if you send out emails to more than ten recipients using To or CC. You can change that in the options, for instance to 1 which would always warn you when To or CC is used.

You can furthermore block emails from being send out at all if the selected value is exceeded.

Thunderbird users can download Use BCC Instead from the official Mozilla extension repository.

The view mode may on the other hand be insufficient for users who work with lots of folders in Thunderbird. Why? Because the unread folders view mode offers no hierarchical structure. All folders containing unread messages are displayed on their own as root folders in the listing.

And that is a problem if you have lots of folders configured in the email client, as it may take longer than it should be to locate a specific folder with unread messages. That, and the fact that you may need to scroll down to see all folders with unread messages.

The Thunderbird extension Advanced Unread Folders adds a solution for users of the desktop email client.

The extension adds a new view mode to Thunderbird that displays unread messages in their folder hierarchy.

You can switch folder view modes with a click on the next and previous arrow icons on top of the sidebar.

Advanced Unread Folders comes with options to always show specific folders, even if they do not contain unread messages. The inbox, templates and favorite folders are always shown by default. This behavior can be changed in the extension’s options.

You can disable the default folders from showing up in the unread folder view mode, or add Drafts, Sent and Trash folders to it.

Some users may only want to show special folders for certain accounts. You can do that by adding those folders to the favorites in Thunderbird and enabling favorite folders to be always shown in the unread folders view mode.

Advanced Unread Folders is especially handy for users with a large folder structure. Users who only work with default folders in Thunderbird do not really need the extension, as the default unread folders view mode handles that scenario well already. The extension is compatible with all Mozilla Thunderbird versions from 3.1 on. It can be downloaded from the official Thunderbird extensions gallery.

The Thunderbird email client does not offer email scheduling options and it requires manual workaround to send out emails at a specific time (save as drafts, send when the time has come, or set Thunderbird to offline mode and go online when you want to send out the emails). It does not work all the time as it requires your physical presence.

Enter the Send Later 3 extension for the Thunderbird email client. Send Later does exactly what it is supposed to do: Schedule emails so that they are send out at a later point in time. Thunderbird still has to be open for that, but you do not need to be present anymore as the emails are send out automatically by the extension at the selected date and time.

The extension provides you with two options. It can highjack the Send button to display a send later menu on every send, or be used via buttons that need to be placed on one of the available composition toolbars of the email client.

The first option needs to be enabled in the extension’s settings. The following menu is then shown whenever the Send button is activated in Thunderbird.

The Send this email later interface may look confusing at first glance. You can specify a time and date, or pick one of the pre-configured options to send the email at a later time. The same menu contains options to send the email multiple times, for instance once per week or year.

A right-click on free space in the Thunderbird write header and the selection of Customize displays the interface element that can be added to the composition toolbar. This is done by dragging and dropping the button set to the toolbar.

Here it is possible to configure a specific date and time for the selected message at which it will be send.

The options offer various configuration settings, for instance to configure hotkeys or the aforementioned option to highjack the send button in Thunderbird.

New users may want to look at the user guide at the developer’s website which explains the basic features and advanced concepts like dynamic values for preset buttons or switching between send and send later button based on the weekday.

Send Later 3 is compatible with all versions of the Thunderbird email client from version 2 to the latest 3.x development builds.

Another excellent feature is the ability to mark text and perform a search or create a link from it. By default Google and Wikipedia are available but the add-on provides access to custom searches as well which can be added in no time in the add-on’s options. Three links can be created from any word that is marked, they lead to www.word.com, www.word.org and www.word.net with the option to add custom extensions like co.uk as well.

Another interesting feature is the option to convert email addresses in the mail body to links. Say someone added a mailto link in the body text of the email. This could be converted into a link to the domain of the email, e.g. from someone@something.com to www.something.com. It can also handle Javascript links.

The option to add custom manipulations is what makes this Thunderbird add-on special. Each function is accessible through the right-click context menu.