Yes, Mozilla Firefox 1.5 is old and outdated, yet it's a notch newer than Mozilla Firefox 1.0. Some people may be stuck with it (when stuck using a Live CD or an older computer with an older operating system). When comparing Firefox 1.5 with 1.0, then it, of course, renders slightly better and with less caveats. For computers with sparse resources, Firefox must still be configured to run efficiently.
And not just: It can handle newer extensions, the greatest of which in order of importance are NoScript and Flashblock, and now add Adblock Plus (ABP). The reason? Some modern (as of 2011) websites now harobur designs that do require collapsing some elements in order to view their pages with some attempted modicum of properness (anything on wikia.com). And Firefox 1.5 is the earliest browser to support a version of ABP that collapses CSS elements by id.
Here's how: Adblock Plus 0.7.0.2 (08.06.2006) is the last to run (to be properly supported) in Firefox 1.0, but most unfortunately, lacks the necessary feature.
The next version (0.7.1), which incidentally first includes the functionality of collapsing CSS elements by their ID, only supports Firefox 1.5 or newer. (But this is not the version of ABP you'll need, if you're somehow stuck using Firefox 1.5.)
The last version of Adblock Plus to support Firefox 1.5 is 0.7.5.5 (01.12.2008).