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My Mac X10.5.8 PCG5. Can't open Firefox 4 'not supported on this architecture' why not?

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When I tried to open Firefox 4 a dialogue box opened stating "Can't open the application Firefox because it is not supported on this architecture"

When I tried to open Firefox 4 a dialogue box opened stating "Can't open the application Firefox because it is not supported on this architecture"

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For older Macs that aren't supported in Firefox 4.0, try TenFourFox for PowerPC's running Mac 10.4.11 & 10.5.8 .
http://www.floodgap.com/software/tenfourfox/