Either upgrade your OS or try another browser that is being kept up to date.
Firefox
Check that your computer is compatible with Mountain Lion/Mavericks/Yosemite/El Capitan.
To check the model number hold down the option/alt key, go to the Apple menu and select System Information.
iMac (Mid 2007 or newer) model number 7,1 or higher
Your Mac needs:
OS X v10.6.8 or OS X Lion already installed
2 GB or more of memory (More is better - 4 GB minimum seems to be the consensus)
8 GB or more of available space
Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.
Application Compatibility
Applications Compatibility (2)
Do a backup before installing, preferable 2 backups on 2 different drives.
If your computer can be upgraded, go to the App Store and downloads El Capitan.