Firefox only: The introduction of Mozilla's Collections was a boon for those that reinstall Firefox often, so they don't have to search out their favourite add-ons every time. Firefox extension Massive Extender makes it even easier by installing collections with one click.
One of Collections' biggest downsides is that even though you don't have to find all of your extensions over again, you still have to install each one individually. Massive Extender adds the ability to install (or uninstall) collections of add-ons all at once. You need to have the Add-On Collector extension installed, as Massive Extender tweaks it directly instead of acting as a standalone add-on. To use it, go to the subscriptions tab of your add-ons window. You can either decide to install all of the add-ons, or use the checkboxes underneath each one to batch install just part of the collection.
Massive Extender is a free download, supposedly works wherever Firefox does — it is still experimental, and it didn't work on our Mac, but we can confirm that it works well in Windows. If you have a Mac and find it to work, or if you have a Linux machine to test it on, let us know how it goes in the comments!