I recently switched to an older notebook and my Chrome and Firefox browsers were quite slow on it. Using the Task Manager in Chrome I noticed that one of the main culprits was AdBlock Plus which was hogging memory.
Searching for a leaner alternative to ABP I switched to uBlock Origin. (Confusingly, there is also a uBlock addon, which is a fork of this.) Just install the addon and you are set! It uses many lists that are maintained online and the blocking is automatic. If you need to turn off blocking or change a configuration just click its button in the toolbar.
I clearly noticed a 50% or more reduction in memory usage with uBlock Origin compared to ABP. The improvement in actual usage was also clear.
Tried with: uBlock Origin 1.1.1, Firefox 41 and Ubuntu 15.04