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Using about:startup
The About Startup add-on provides a much simpler way to measure startup performance. Once installed, it will gather startup data and display it in the about:startup page. The add-on is very lightweight, so it shouldn't have a noticeable negative effect on your baseline and add-on tests.
To test with about:startup, you should set up a new Firefox profile, install this add-on, and then open and quit Firefox repeatedly. If you want to match Talos, you should do this 20 times. Then look at the results in the about:startup page. This should provide your baseline numbers. Then repeat the process with both About Startup and your add-on installed. The overhead can be calculated using the same formula.
The about:startup page shows 3 different time measurements: main, sessionRestored and firstPaint. main happens very early in the startup process and should not be affected by add-ons being installed. The firstPaint time should be roughly similar to what Talos tests. This can vary between systems, though. sessionRestored is fired later in the process, when Firefox has loaded all tabs from the saved session.