The only thing I've seen is from the Add-Ons FAQs and it really doesn't adress the question.
Can add-ons make Firefox slower? Most add-ons do not cause a perceivable performance decrease in Firefox, though installing an excessive number may have adverse effects. If you suspect an add-on is causing Firefox to be slow, try disabling it. Add-ons that cause large start-up performance issues will have warning messages on their details page.
I would guess a lot depends on basic computer specs like RAM, CPU, etc. Almost like asking how many startup processes can Windows 7 handle before it starts to slow down.