If you want to keep it practical for regular use:
Check how much RAM you actually have. It's not 8 MB; that's the video RAM. Assuming it's not maxed out to 1 GB, you can go buy some on eBay for a pittance. Then burn a copy of Mac OS X 10.3.9. This is the latest version of Mac OS X supported by your model and will let you run a reasonably newish browser like Camino 1.6.11, a bugfix update released just last year, or if you prefer, Firefox 2.0.0.20 (if you want to use extensions), although that's from 2008.
Or, you may instead wish to install Linux. The latest version of Ubuntu, 10.10, is still being maintained for PowerPC. Running Linux will let you use Firefox 5.x and also strip out anything from the system you don't want/need. Running Fluxbox instead of Gnome, for example, will make that speed along nicely.
>FF behaves really weirdly on MacOS
Yep. Remember when we had Camino? It integrated so nicely - like Safari but on Gecko. /sigh
>Btw the bug was signaled 1 year ago.
It's not an easy problem to solve. There is someone actively working on it though (although they appear to be on vacation right now).
The gfx.compositor.glcontext.opaque
workaround makes a huge difference if you're willing to have a slightly less pretty browser.
It looks like Camino, which stopped development in May 30, 2013. Assuming the browser wasn't used after development stopped then it may have been latest Winter 2012.
In my opinion the best Mac web browser. It has no addons or frills, but blocks pretty much all ads and popups and does a better job than anything else of stopping websites messing with my browser window. I like a fairly simple web browsing experience and Camino is perfect for me.