It depends on many factors To rule out are browsers that work with Cryptocurrencies, as they need dedicated servers that consume additional energy (Brave) Browsers that do not have an ad and trackerblocker, since these do not impede the bandwidth used by these ads, which is also an extra expense. They don’t use servers for sync and services that run on renewable energy. That increase resources and energy with many tabs open. For the user, preferably use dark themes. In an LED screen, black means all the pixels off, which saves energy, since then only the backlighting and the text and images on the screen use energy, apart better for the eyes. Using search engines like Ecosia, which uses renewable energy and also plants trees with searches, also protects privacy. Apart from SeaMonkey, I use the Vivaldi Browser from Iceland, it use Green Energy, can hibernate tabs and include Ecosia as one of the default search engines and sponsor. I use Seamonkey only occasionally, depending on Mozilla and using the servers of this company, funded by Google and Alphabet Inc, which is far from being ecological. Few alternatives remain.