I hope you have already found the zoom controls and other features

• Font size and zoom - increase the size of web pages
• Accessibility features in Firefox - Make Firefox and web content work for all users
  • http://accessfirefox.org/

    Access Firefox?
    To sum it up, we provide:
    * Accessibility tools and resources for Firefox end users with disabilities (primarily visual impairments)
    *Firefox introductory materials for those new to Firefox, or curious about it
    *Firefox tips and guides for all Firefox users (from beginners to power users)

Your webmail is dependent upon whatever service you use and their own sites provide support and instructions see

Problems with email and how to find help

The text immediately above is probably in blue and is a clickable link

Update.

Sorry I replied thinking the question was about the Firefox Web browser. You may use that or consider doing so in which case the information may be helpful.

Your question related to Thunderbird so I should have just suggested looking at this:

• Sending and Receiving Messages on Thunderbird

Cannot delete messages

You could also be interested in Thunderbird add-ons - frequently asked questions There are addons that allow modification of Thunderbird for instance https://addons.mozilla.org/en-US/thunderbird/addon/stylish/?src=hp-dl-mostpopular

There are several method you can use. Choose the one that you are most comfortable with.

When you select an email to read - a single left click on email in list. It will display in the lower Message Pane. A double click will open the email in a tab or window.

1. At the top you will see all the headers: FROM, SUBJECT, TO In this same header area there should be a 'Delete' button.

• click on 'Delete' button to delete the message.

Alternative methods : 2

• single left click on email in list to highlight the email.
• Press 'Delete' key on keyboard.

These next two methods can be used to delete without opening the email. 3

• right click on email in list and a drop down menu appears.
• single left click on 'Delete Message' will delete the email.

OR 4

• right click on email in list and a drop down menu appears.
• Press the letter D on keyboard.

The email should be removed from the folder and reappear in the 'Deleted' folder which is sometimes called 'Trash' or even 'Deleted Items'.

How to empty the 'Deleted' folder.

• Right click on 'Deleted' folder and select 'Empty Deleted'.

Periodically you need to compact folders. This removes all traces of deleted emails. File > Compact folders or Menu icon > File > Compact folders

You can compact a single folder: right click on folder and select 'Compact'