Mac Pro - web browser scroll speed slow

I have a Mac Pro 2008 Xeon/OS X 10.8 with 8 cores at 2.8 ghz. It has 14 GB RAM and a Radeon 5670. The boot drive is a 3G SSD from OWC. 25 MB broadband Internet connection. Pretty solid configuration, right?

However, image-heavy web pages don't scroll as quickly as my 2012 MacBook Air. Web page scrolling bogs down, I frequently find myself waiting for pages to render. For example,

Generally, the Mac Pro feels slower than the little MacBook Air, but the web page scrolling is particularly egregious.

Both Macs are running Firefox 23 with AdBlock and the Ghostery plugin, same config. Firefox on the Mac is pretty slow but I'm comparing apples to apples, so to speak.
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Do you have the same problem if you use Safari?

Have you tried temporarily disabling your plug-ins on Firefox?

Anything in the console logs to indicate the problem?

Might it be a network problem?

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suggests opening the Firefox 'Add-Ons Manager' and removing out-of-date add-ons and disabling plug-ins.
d4nnyoAuthor Commented:
Speed much faster after updating/removing plugins
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