I struggled with this for months trying to find an answer. The FF options settings stopped working around 188.8.131.52, I think. But this worked for me to get the visited links in red.
Copy this to you userContent.css or create the file first if you have to. It should be in your profile, chrome folder.
|/* Change Visited Links to Red */
color: #ff0000 !important;
My visited links now stay red. I can keep the website design with it. And the online updates from 184.108.40.206 to 220.127.116.11 didn't break it.
If you want to use purple, I guess you could change "Red" to "Purple" by finding the color number in Thunderbird if you're using it. I think that's where I found a number for red a long time ago.
Also look at your options tabs for:
WEB CONTENT-SETTINGS FOR FONTS AND COLORS
Uncheck: Allow pages to choose thier own fonts, instead of my selections above
Unvisited Links: Blue (or your choice)
Visited Links: Red (did it here; but now it doesn't work until the userContent file is in place.)
Check Underline links (or not...your preference)
Check Allow pages to choose their own colours, instead of my selections above. (This keeps all the graphics etc in the page instead of whiting out to just a page with text on it.)
These settings alone in FF Options used to work in early versions of FF without changing the user.Content.css. But a long time ago they stopped working by themselves.
I haven't updated to FF 2 yet so I don't know if that will break it. So I'll keep your hint on file in case it does. Also, in my searching for an answer to this, the number of days you keep for your history might also be a factor. For example, I keep 4 days of history. I've noticed when going back several pages in a forum (PG is one of them), threads I know I've been to are now back to the original blue color.