I need to run Gmail and Google Calendar in a separate browser instance from my working browser so they are isolated from crashes/hangs. I tried the Firefox 3 Prism plugin along with gmail.webapp and gcalendar.webapp but wasn’t happy with this - mainly because I don’t want a separate instance for gmail and calendar. I want my browser and one other instance hosting both gmail and calendar.

You can run multiple Firefox instances by directly launching /Applications/Firefox.app/Contents/MacOS/firefox - but risk corrupting shared profile database files.

So I ended up creating a simple profile just for these webapps - launching the Profile Manager with /Applications/Firefox.app/Contents/MacOS/firefox -P -no-remote and creating a new profile named google.

Now I can launch gmail and calendar using this new profile in a separate Firefox instance like this: /Applications/Firefox.app/Contents/MacOS/firefox-bin -P google -no-remote https://mail.google.com/mail https://www.google.com/calendar/render. I packaged this shell script in a bundle as Google.app and gave it the icon borrowed from Google Notifier.app. You can download it here.

Also, if you prefer monospace fonts when viewing and composing plain text emails in Gmail, create a user CSS file in the google profile you created above (e.g. ~/Library/Application Support/Firefox/Profiles/*.google/chrome/userContent.css) with the following contents:

@-moz-document domain(mail.google.com)
{
    /* GMail messages and textarea should use fixed-width font */
    .ArwC7c, .iE5Yyc {
        font-family: MonoSpace !important; 
        font-size: 9pt !important; 
    }
}