OK, thanks for that. I forgot to ask you a few more questions:
1. Are you making backups to an external device? That is so, so critical. If you (hopefully) are, what software do you use?
2. How did you "arrive" at Sierra? That is, when you upgraded, did you upgrade "in place", ie, have the initial version of Sierra basically "over write" the OS you previously had?
3. When you did disk cleanup/maintenance/repairs:
A. What software/tools did you use?
B. Did you do those tasks in an isolated manner, ie, booting your Mac from another "source"? Most folks rely on booting to the (hidden) Recovery HD partition, and running Disk Utility from there. That's fine. Myself I do a good amount of disk cleanup on my own. I also use the excellent freeware program called Onyx. And I use the excellent commercial program Tech Tool Pro (which has an eDrive feature, akin to the Recovery HD partition created by a clean installation of the Mac OS). Finally, in removing an application, I use the freeware program AppCleaner, which gets rid of just about all the associated files of the application (besides of course the application itself).
4. What other third party software do you use?
From what you are describing, and especially since you have similar issues with the venerable program VLC (thus not using the internet to view videos; I use VLC also), it might be a good time to do a clean, fresh, "virgin" installation of Sierra, OS 10.12.6. However, if after such an installation you want to "migrate"/copy needed "stuff" from a backup, that might include whatever is causing your issues. Do you have a lot of data files on your machine that you absolutely need? Also, depending on which third party apps you use, if you don't have many data files (and can "live" without them), and if the number of third party apps is small, you could just "migrate"/copy your account "stuff" from the backup, and then after restarting your machine, install each app one by one (Firefox, Google Chrome, VLC, etc.).