on windows machine, one clicks on the "tools" pulldown menu select "options" at the bottom of that menu. in the middle button on the left hand side of the window is a globe icon with "web features" under it. That is where one finds the checkbox for popup blocking, although it is already checked by default.
for mac it is under the "firefox" pulldown menu under "preferences"
but now that I type this, I realize I am using 1.0.2, not 1.5. I dunno if the new version is different. I guess I should check that out.
eta
opps. the button with the globe is now in the middle at the top of the window and just says "content" on version 1.5