On first startup in a new profile, system addons are synchronously "installed" (the addons themselves are read from the application directory but local profile state such as addonStartup.json has to be created) early in startup. This is particularly expensive for screenshots since it bundles its translations for all supported locales.
There are lots of ways we could chip away at this but I think our best option is to use the built-in addon capability added in bug 1512436. As a first cut, we could simply move screenshots into omni.ja and, on first run, install it asynchronously after the browser UI has been painted. Note that we'll also need to handle updates when the application updates.
As a separate step, if we moved the screenshots strings out of the extension and into language packs, that would make the install/update process less expensive (and maybe it would make the l10n team's work easier by making it follow the same process the rest of the browser uses). That would entail dusting off the patches from bug 1425104.