plugin-container has high CPU usage and memory usage, how can I check further for problems?

Top Processes by CPU:

58% plugin-container

28% firefox

13% quicklookd

8% kernel_task

7% WindowServer

Top Processes by Memory:

2.25 GB plugin-container

1.52 GB kernel_task

1.30 GB firefox

467 MB Mail

189 MB com.apple.WebKit.WebContent

How can I look into this further and resolve any problems I might find. If I've got some rogue plugins, how can I eradicate them?

Thanks.