While you type in the address bar, Firefox can be doing any or all of the following:

• asking your default search engine for search suggestions
• looking for matches in your bookmarks
• looking for matches in your browsing history
• looking for matches in your search history
• looking for matches among your open tabs
• (in some regions) looking up suggestions via a Mozilla web service or local file

You can turn these different types of suggestions on/off and see whether you can link the problem to a particular type of suggestion. More info in this article:

Address bar autocomplete suggestions in Firefox -- click into the "How can I control what results the address bar shows me?" section

Any difference?

Thank you for the reply. I have tried just that: disabled search engines and all suggestions from address bar. Unfortunately, no difference: typing in address bar still causes CPU usage to spike to 15%

Hmm, what else could it be?

Could you try changing the setting in the "Speculative pre-connections" section of this article to prevent anticipatory connections (not that these should use much CPU, but ??): How to stop Firefox from making automatic connections.

Also, if turning off browsing history suggestions didn't also disable the in-URL-bar autofill behavior, there is a separate preference for that:

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

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future. Since we have had this setting for a long time and I use it myself, I feel comfortable mentioning it.

(2) In the search box in the page, type or paste browser.urlbar.autoFill and pause while the list is filtered

(3) Double-click the preference to switch the value from true to false

Thank you for more suggestions. Sadly, they did not help either.