From Bugzilla Helper:
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:0.9.9) Gecko/20020408

Description of problem:
Mozilla will open a browser window with a blank page even if 'home page on
startup' is selected in preferences if there is already an instance of Mozilla
running.  This is correct behavior if the previous instance of Mozilla is a
browser window, but it will also exhibit this behavior if the previous instance
is a Mozilla Mail window.  The determination as to whether a home page should be
loaded or not should be based on the number of browser windows open, not on the
number of Mozilla instances already running.

AFAIK this is a design issue, not an implementation bug.

Version-Release number of selected component (if applicable):


How reproducible:
Always

Steps to Reproduce:
1. Close all Mozilla browser windows.
2. Open Mozilla Mail.
3. Select Edit->Preferences->Navigator from the menubar, enter a URL for  a
homepage, and select the home page on startup radio button.

4. Open a Mozilla browser window.

Actual Results:  Mozilla will open a browser window with a blank page.

Expected Results:  Mozilla should open a browser window and display a home page,
since this is the first browser window opened.

Additional info:

mozilla-0.9.9-7