My Power Macintosh 8600/250 (damn Amelio and his terrible product names...) running Mac OS 9.2.2 can browse pretty well with Classilla. Even loads Facebook and can handle messaging etc.
IE is a meh experience however...
I tried Classilla but it said I didn't have something installed when I tried to open it... I wish I remembered what it was. Did you ever have a similar issue?
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My PowerBook G3 works comfortably, but my Macintosh SE can't even load a single picture. What do you think is the oldest Mac (with upgrades) that can surf the internet?
A friend of mine has an upgraded Macintosh Color Classic. It can surf the web with images, but obviously it can barely load most things. Sadly the internet is full of junk today. We could still browse simply if things were still simple.