I'm on MacOS Mojave 10.14.6 with Firefox Quantum 69.0 (my browser of choice 
I have several tabs open in the browser. Whenever I click on the tab that shows a page on Confluence Cloud with the Viewpdf-macro on it (view- or edit-mode, doesn't matter) there's a "FirefoxCP Web Content"-process that goes up to 200-250% CPU. When I click on another tab with no Viewpdf-macro included (e.g. a news site) the CPU-usage of said process drops almost immediately to 5-10%.
It does not seem to be a problem with a particular PDF-file. I can reproduce the behaviour on several pages that each have another PDF on them. The viewpdf-macro does seem to have to work harder for particular PDF-files (in that case the typical Mac "spinner" is shown more than the actual content of the file), but even for pages where the macro has no problems rendering the PDF the CPU-usage still goes way up (as does the fan noise).