First, you should find out where the bottleneck is.
For example, next time it feels like Firefox is slow, open Task Manager (if you are on Windows), and switch to Performance tab:
Is the CPU maxed out? (something is wrong!)
Is Memory almost full? (most common issue, you likely need more RAM!)
Is Disk usage 100%? (something is wrong or RAM is full)
I would guess, your RAM is at 90%, at which point it swaps content to disk, making everything super slow. These days it’s best to have at least 16GB RAM, but for heavy tab user I would go even further to 32GB.
The good news is, RAM is quite cheap these days and many Notebooks and most Desktops can be easily upgraded.
PS: did you know that a single Gmail tab can consume ~1GB of RAM? 