Every time I start Firefox it tells me that all of my add-ons are making Firefox run slow and then never again. I think it makes sense to ignore the first timing for an add-on. Add-ons are going to do most of their work setting up during startup or when enabled and as long as they don't do this longterm that's less of a problem. We should ignore the first timing for an add-on to skip this.

This sounds right to me. Alternatively we can time start up time separately with a different limit to try to catch add-ons that are truly slowing down start ups significantly.