Fix locating firefox on OS X
Fix locating firefox on OS X

Some mac installations don't link firefox into '/Applications', (e.g. homebrew links into '~/Applications'). A more reliable method is to use the app id for getting the path, (i.e. 'mdfind kMDItemCFBundleIdentifier==org.mozilla.firefox').
For path fallbacks, '~/Applications/...' added in addition to '/Applications/...'.
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