I'm running a machine with the same specs as you and I don't ever experience Firefox bogging down as you are describing. I do believe Firefox is more heavy-weight than Chrome, but nothing to the point where I have to consider updating my OS or running an SSD. I set my browser to delete everything when I close and launch again. This helps to keep registry and temp files from building up. Perhaps this might help?
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I'm running a machine with the same specs as you and I don't ever experience Firefox bogging down as you are describing. I do believe Firefox is more heavy-weight than Chrome, but nothing to the point where I have to consider updating my OS or running an SSD. I set my browser to delete everything when I close and launch again. This helps to keep registry and temp files from building up. Perhaps this might help?