GAM1 said

How do I disable display of the list of "Frequent" websites in Firefox?

Hello GAM1,

Assuming that you are talking about the jump-list of frequent used web pages when you right click the Firefox taskbar icon:

Type in the address bar about:config and press Enter. (ignore the warning)

Type in the search bar and look for the preference :

browser.taskbar.lists.frequent.enabled and set its value to false.

Then close and restart Firefox.

I hope this does what you are looking for ....

In order to install a downloaded extension, please do the following:

1. Click the menu button and choose Add-ons.
2. Drag your downloaded XPI file to the Add-ons Manager tab and drop it.

or

1. Click the menu button and choose Add-ons.
2. Click the gear icon at the top-right corner to the left of the search box and select Install Add-on From File…
3. Locate the XPI file you downloaded and click Open.

You may need to restart Thunderbird when prompted. For more detailed instructions and information about installing add-ons, see Installing an Add-on in Thunderbird.