ok, I tried removing all of the xmodmap and imwheel commands from .xinitrc and manually starting them from the command line once I was in X. Here are the results:

$ xmodmap -e "pointer = 1 2 3 6 7 4 5" <--- clean, no errors

$ imwheel -k -p -b "0067" -f

WARNING: imwheel pid file could not be removed.
Perhaps you want the -p option.
Or chown root and chmod u+s the imwheel executable...
: Operation not permitted
WARNING: imwheel is not checking/writing a pid file, BE CAREFUL!
An imwheel may be running already.
Two or more imwheel processes on the same X display,
or simultaneously using a wheel fifo,
will not operate as expected!
INFO: imwheel started (pid=9047)

so I ran

# chown root /usr/bin/imwheel && chown u+s /usr/bin/imwheel

$ killall imwheel
$ imwheel -k -p -b "0067" -f
WARNING: imwheel is not checking/writing a pid file, BE CAREFUL!
An imwheel may be running already.
Two or more imwheel processes on the same X display,
or simultaneously using a wheel fifo,
will not operate as expected!
INFO: imwheel started (pid=9130)

so now what? I commented the imwheel command out of .xinitrc and I "killall imwheel" before I run imwheel again.

hmmm running out of ideas!
your terminal error msgs are the same as what i get, only mine works!
youv'e got the same Xconf, .xintrc, and imwheelrc as me... and the same imwheel msgs. :|
the only differences left as far as i can tell, is that im running the std mandrake 9.2 imwheel, version 9.9, with imwheel "67" instead on "0067".
Oh yeh and my mouse is an MX 700 not 500, but that dosent matter because your buttons are registering the same in xev.

but the fact that your back and forward worked for eveything else, just not firefox... stil sounds like an imwheelrc issue.

But, i dont know any more, you said you'd tried all that "(null)" stuff.
Sorry

Yes, I'm the guy from the other thread.

Try running imwheel -k -D -b 0067 from an eterm. When you click on a window, say firefox, it will show the debug of imwheel. This helps greatly when troubleshooting imwheel. Here's my relevant output when I click the back button (button 6) on the mouse.

WinAction (0x8058390):
Priority : 0
Window Regex : "(null)"
Keysyms Mask (0x8058340):
"None"
Button : 6
Keysyms Out (0x8058360) :
"Alt_L"
"Left"
Reps: 1
Rep Delay: 0
Key Up Delay: 0

This shows I click the firefox window, (null), with button 6, which translates into Alt_L + Left. What are you getting?

button 6 output
---------------------
Keymap status:

XModifierKeymap:
[0,00] 50 0x32 "Shift_L"
[0,01] 62 0x3e "Shift_R"
[1,00] 66 0x42 "Caps_Lock"
[2,00] 37 0x25 "Control_L"
[2,01] 109 0x6d "Control_R"
[3,00] 64 0x40 "Alt_L"
[3,01] 113 0x71 "Alt_R"
[4,00] 77 0x4d "Num_Lock"
getInput: Button=6
ButtonPress:window=0x00e0002c "LinuxQuestions.org - extra mouse buttons in Firefox 0.8 - where Linux newbies come for help - Mozilla Firefox"
resource name ="Gecko"
class name ="Firefox-bin"
ButtonPress
Taking default action.
makeModMask: 0x0
keycode=100
rep=0 key='Left'
ButtonPress:window=0x00a002c2 (null)
resource name ="(null)"
class name ="(null)"
ButtonPress:window=0x0120003f "schbond@wwwyzzerdd:~"
resource name ="Eterm"
class name ="Eterm"
ButtonPress:window=0x00e0002c "LinuxQuestions.org - extra mouse buttons in Firefox 0.8 - where Linux newbies come for help - Mozilla Firefox"

button 7 output
---------------------
Keymap status:

XModifierKeymap:
[0,00] 50 0x32 "Shift_L"
[0,01] 62 0x3e "Shift_R"
[1,00] 66 0x42 "Caps_Lock"
[2,00] 37 0x25 "Control_L"
[2,01] 109 0x6d "Control_R"
[3,00] 64 0x40 "Alt_L"
[3,01] 113 0x71 "Alt_R"
[4,00] 77 0x4d "Num_Lock"
getInput: Button=7
ButtonPress:window=0x0120003f "schbond@wwwyzzerdd:~"
resource name ="Eterm"
class name ="Eterm"
ButtonPress
Taking default action.
makeModMask: 0x0
keycode=102
rep=0 key='Right'

OK, looks like imwheel recognizes YOUR firefox window. Change "(null)" to ".Firefox"

haha im glad you came! i thought id take my new found knowlage and help out, but i guess i didnt really succeed
/me gracefully bows out and lets cexshun advise properly

btw, thanks again cexshun, i cant live without those side buttons

Easy man, don't have it fixed yet...

yes, and I made this change, but still no back/forward buttons in Firefox. Somehow along the line editing all these files, I don't have back/forward in ANY file/web browsers. Instead they are side scrolling buttons.