I find, on my own system, this is caused by other tab add-ons like those for custom tabs such as blank tab, etc.

I would like to suggest a start-up delay time be added to Options if possible. Perhaps wait 5-10m after Firefox opens to begin updating dials.

It's pulling the info from the Add-on library description title.

You can now turn off the Show Page Action button in Speed Dial > Options > Appearance.

Go into Firefox about:config and change browser.newtabpage.activity-stream.enabled to false. This also changes new tab back to classic. Then install a custom tab Add-on that loads a blank tab or other custom tab.

Note: A side effect of doing this will cause the Speed Dial URL to load in the address bar field on the Speed Dial Home page.

If you have Firefox 57 and this happens, I find a custom new tab Add-on can cause it.

Speed Dial URL can still persist in the address bar field when there is a load custom tab Add-on installed in Firefox. When tab Add-on is removed, the address field does not contain Speed Dial URL.

To add an FYI about Speed Dial while in Private browsing:

Speed Dial persists to load in new tabs while in Private browsing, so you wish for new tabs to load as Private without Speed Dial, then there is a need to set browser.newtabpage.activity-stream.enabled to False in about:config. This sets the new tab behavior to classic. Then use an add-on that sets the new tab to a blank page. Unfortunately, this also causes Speed Dial URL to appear in the address bar field on a non-Private Home page.

Also suggest to move any Flexible Space off the bar. In addition, the Address bar field can be dragged to the right to lengthen. Finally, the Show Page Action button can be disabled in Speed Dial options on the Appearance page if you wish to remove it from the address bar field.

I would suggest when your new tab is loaded with Speed Dial, go to Firefox Options and set the current page as your Home page.

See this thread in this forum for sizing suggestion.

If you are running on a large monitor, I understand dial size may be a concern.

If you want to set your homepage back to Firefox default, disable or remove Speed Dial extension then go into Firefox Options for Home page and select Restore to Default.

You can drag and drop dials where you want them.