Are you referring to the form history that displays in a drop-down menu when you click a text field?

If you would like to delete a single entry in the drop down menu, you can hover over the item on the drop down list and press the delete key on your keyboard.

Alternatively, you can also try installing the Form History Control addon for additional control over what form entries are saved in Firefox.

If you would like to delete all of your Firefox form history, please read this Mozilla support article:

• Delete browsing, search and download history on Firefox

I hope that I was able to provide you all of the information that you require. Please let us know if you require further assistance or explanation.

This did not work. The subject line when you compose has a drop down list that you can save frequently used subject lines. I get the mouse over the the line in the drop down box--it is highlighted and hit the delete key on the keyboard and nothing happens . No deletion.

I apologize. On some operating systems, you will need to press Shift + Delete when hovering over the entry.

This did not work. I am talking about when you open the window to write an email. Next to the subject line of the the outgoing email is a drop down box. I have save some common subject lines I have used for projects. I want to delete some of them. What you are describing works if you are just looking at the inbox of your email and want to erase an email in the inbox. I am talking about the window that opens when you want to write an email and you use a subject line.. I am using Windows 10.

Firefox doesn't do email, it's strictly a web browser.

If you are using Firefox to access your mail, you are using "web-mail". You need to seek support from your service provider or a forum for that service.

If your problem is with Thunderbird, let us know and we can move this thread to the Thunderbird queue. This question currently is in the Firefox queue for answers.

This is a Thunderbird issue