You cannot change how many are displayed in the back/forward drop down at a time.

You can change the number but not how many are displayed at a time in the dialog window. Select the earliest entry you can on the back button (the lowest entry), and then invoke the Back button again, and go to the earliest entry.

I have test data you can try: I have browser.sessionhistory.max_entries set to 70 the default is 50

To test bring up

• http://dmcritchie.mvps.org/firefox/tab_capacity/001.htm

What you see on the page are a lot of numbered links 001 - 060

1. start with the 001 entry in a new tab make sure you started it in a new tab
2. then go successively through 002-060 in the same tab then go up into bogus numbers to 70
3. Right click on the Back button, you will see that you can see 15 entries at a time
4. Click on the earliest visible at the bottom of the heap
5. Click again on the Back button to select the earliest again
6. Repeat previous step until to see 001, all entries were accessible through the Back button, but 15 is the maximum that you will see at a time..

My problem is the max of 15 at each time, not the total of entries. Adding a config to change it in about:config would be interesting.