Right now I’m trying to keep my drop down history from moving my standard links around. I have a set order of sites and I like to keep them in the order I go to them.
The sites shown and the order they are shown is a complex algorithm. You can read the following, but I advise staying away from the configuration settings if you do not have time to experiment over and over and over. This was written some time ago and may have changed a little. Some details here (read “Frecency (misspelled) Calculation” and “Configuration”): https://wiki.mozilla.org/Firefox/Namoroka/Improved_form_history#Frecency_Calculation
More: https://support.mozilla.org/en-US/questions/856200