Linnes said

On a daily basis I get these errors:
A web page is slowing your browser - Server not found - Firefox not responding - Stop script or continue

These could be part of the same set of symptoms, or they might be unrelated.

• A web page is slowing your browser + Stop script or continue. As a starting point, see: Warning Unresponsive script - What it means and how to fix it

• Server not found. Does this persist after a reload? or if you reload (using the button in the address bar or Ctrl+r) does the page load?

• Firefox not responding. As a starting point, see: Firefox hangs or is not responding - How to fix.

Regarding Flash, as long as it is set to Never Activate in Firefox, I don't think you have to worry about the leftover files.

If there is a pattern of particular sites causing problems, that may be useful to know. There's also this general article: Quick fixes if your Firefox slows down.

Ive tried all suggestions above and nothing has changed. What else can I do?

I really need to solve this soon.

First, I want to mention that significant parts of Firefox are being overhauled in the Firefox 57 release in a few weeks, and performance will be much better on average, so you should get some relief from that.

But for right now: have you considered reducing the amount of stuff Firefox loads/processes by using an ad blocker or tracking blocker? These are some popular choices:

• https://addons.mozilla.org/firefox/addon/ublock-origin/
• https://addons.mozilla.org/firefox/addon/privacy-badger17/