Because Webdriver waits for the entire page to load before going on to the next line, I think disabling images, css and javascript will speed things up.
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The accepted answer doesn't work for me either. From the "reason" links referred by kyrenia I gathered that Firefox overrides the "permissions.default.image" preference on the first startup and I was able to prevent that by doing:
# Arbitrarily high number
profile.set_preference('browser.migration.version', 9001)
Which seems to be ok since I create the profile on each driver startup so there is nothing to actually be migrated.