The NPAPI plugin mechanism is the standard mechanism for browser plugins on MacOS (and Linux - everything except IE) - if you use NPAPI, your plugin will work in Safari, Firefox and Opera. They will also work on both 32 and 64-bit Safari. Assuming your code makes no assumptions about which browser it runs in the same NPAPI code, it should work in all browsers (I saw "NPAPI" plugins that dynamically enable XUL-related functions in the blind belief that NPAPI is only used Firefox, despite this, is the standard plugin format for more or less every browser that does not use IE).
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