There really are numerous reasons for why things may be slowing down after an upgrade, an update is merely a new version of what you already have, an Upgrade is a different operating system entirely.
So if it is just your browsers that have slowed down I would suggest that is due to either network speed or a compatibility issue with the applications in question. The best means of elimination here would be to compare Chrome and Firefox to Safari. If Safari is also very slow it could be due to a temporary network demands from Spotlight which may be Reindexing your data or iCloud downloading backed up data such as contacts, emails, photos, music ect.
These are only guesses though because we do not know how you upgraded, eg clean instal, upgrade over existing macOS or onto what device.
So please get back to us with the info from About This Mac under the Apple Menu (we don't need your serial number) Just Model, Year and Memory and remaining free storage from the Storage tab on the same window.