”Webpage is slowing down your browser” This is the new way the browser tells you there is a problem with the script program(s). https://support.mozilla.org/en-US/kb/warning-unresponsive-script?cache=no

Edit :

While I was typing my answer, FredMcD posted his reply, which is pretty much the same as what I wrote, so I removed my post.

FredMcD said

”Webpage is slowing down your browser” This is the new way the browser tells you there is a problem with the script program(s). https://support.mozilla.org/en-US/kb/warning-unresponsive-script?cache=no

That's not true. The script warning is a separate warning, and turning it off doesn't turn off the warning about pages slowing down your browser.

Would you please do this :

Type in the address bar about:config (press Enter) (promise to be careful, if asked)

Type in the search bar

processHang

and look for the preferences :

dom.ipc.processHangMonitor and dom.ipc.reportProcessHangs

Set both values to false

Then close and restart Firefox, in order for the change to take effect.

I did the above. Now I'm getting a whole new, much more obtrusive warning... a huge modal drop-down sheet telling me when an extension is making the browser slow.

Honestly, these "solutions" are just different iterations of the problem. The browser needs to stop interrupting me to tell me I'm being temporarily interrupted. There needs to be a way to turn all this garbage off. It needs to stop freaking out every time a script thinks hard for a minute.

I only recently switched back to Firefox after a long time away and I have to say it really seems like they've ruined it. It's a total pain to use now, largely because of things like this.

Look, less than a minute after the above post, it just happened again. I never saw this before I took the above advice, and now, I'm getting this every 2 minutes. I've used Evernote on other browsers for years without having to deal with this nonsense.

As a test, disable Evernote. If the problem goes away, contact its support.

Thank all for answers. My problem is gone. I don't know when but after I simply cleared cache and cookies. And reproduce situation with browser still I can't.

Might have been a corrupt file that was removed. Please flag your last post as Solved Problem so others will know.