Hi Hugo,
Please use Firefox version 10.0 or later and run it in 32bit mode. (via Applications->Cmd+click on Firefox icon->Get Info->Check Open in 32bit mode)
For Safari, if your Safari version is 5.1.x then please run it in 64bit mode.
For your information:
With its latest release (i.e. 10.1.3) Adobe has dropped support for all versions of Firefox prior to 10.
For more details see: release notes
hope that helps,
rohit