Let's start with Kaspersky's feature of scanning your web traffic. This article describes how you can disable it temporarily to see whether that is the problem: How to troubleshoot security error codes on secure websites.

If that's not the issue, could you copy/paste more detail from the error page? If you click the Advanced button, there should be some more technical info in that section.

For some of the other issues:

• An ad-blocker reputedly lighter on resources than Adblock Plus is this one: https://addons.mozilla.org/firefox/addon/ublock-origin/

• Sometimes page content may be blocked by Firefox's tracking protection feature, especially if set to its strictest setting. This article covers how to manage it: What happened to Tracking Protection?.

Please see the section about Kaspersky in this article:

• https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER

Note that Adblock Plus has been reported to cause performance issues.

You can look at uBlock Origin as a replacement for Adblock Plus to see if that works better.

One of the main technical changes in Firefox 54 was to enable more individual content processes. In Firefox 48-53, the Firefox multiprocess feature would create one process for the UI and one process for content. Now Firefox will aim for four content processes. Perhaps this is related to the reduced performance?

You could try rolling back the number of content processes to 1. If this doesn't work, you may need to revert to single-process mode. Here's how you can try this:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste ipc and pause while the list is filtered

(3) If the dom.ipc.processCount preference is bolded and "user set" to 4, double-click it and edit the value to 1, then click OK

Presumably that will not take effect until the next startup, at which time you should check it to make sure it hasn't reverted back.

Any difference?

Alternately, you can disable multiprocess mode using a different preference:

(4) In the search box above the list, type or paste autos and pause while the list is filtered

(5) Double-click the browser.tabs.remote.autostart.2 preference to switch the value from true to false

Note: the exact name of the preference may vary, but it will start with browser.tabs.remote.autostart

At your next Firefox startup, it should run in the traditional way. Any difference?

Thanks so much for the rapid replies. I'm in the process of trying your suggestions-will post if successful. The settings for dom.ipc.processCount was not bolded and set to 1. Trying the alternative now to switch true to false.. I sent message to Kaspersky -their solution was to add URL of problem site to "trusted"-I did. No change. I started FF in safe mode with all addons disabled-No good. Removed Adblock plus. So far, I can only read/post if I pause KS protection while on site. Not an ideal solution. Re : tracking protection-do you advise turning off altogether or only in protected mode?? Many thanks -fingers crossed !

"Let's start with Kaspersky's feature of scanning your web traffic. This article describes how you can disable it temporarily to see whether that is the problem: How to troubleshoot the error code "SEC_ERROR_UNKNOWN_ISSUER" on secure websites." This seemed to solve my problem with reading/posting. After last update-54.-FF continues to be extremely slow-both in start-up and operation. After further reading on KSM, discovered that KSM is known to cause performance issues. I unchecked the "insert script.." box in settings and seems to help-somewhat.. Thanks much to all ! Regards.

sckat said

After further reading on KSM, discovered that KSM is known to cause performance issues. I unchecked the "insert script.." box in settings and seems to help-somewhat.

Is KSM a feature of Kaspersky? Not sure I've heard of that one before. If that continues to help, could you give a little more detail about it?

Nah- Since I'm a 1-finger typist (a huge misnomer, that), KSM is just quicker to type than KASPERSKY. Suppose I shoulda/coulda written KSP. I followed suggestions on the Kaspersky (2017) help section on optimizing computer performance. Don't think we'll use this after our subscription expires. BTW-took all your suggestions.. As stated, I know just enough to get me into a fix.Regards. http://support.kaspersky.com/13157 ###