You might want to look at the Activity Monitor to see how much resources Avast is taking up. Also, version 38.0.5 of Firefox came out the other day.
Assuming the same slowness (or similar) would happen if you disabled Avast from running, then most likely, the slowness is due to issues with your internal drive. They are either software related, hardware related, or both. As it is, assuming the hard drive is the original one, it is already 7 years old, and hard drives "tend" to start and get problematic at their 5 year point. But, that is not a "hard and fast rule", per se. It depends upon usage of the drive, whether any disk repair/maintenance, etc. has been performed, etc. Additionally, that "stock" drive spins at only 5400 rpm, so it is going to be somewhat slow anyway.
So, to determine the "health" of the hard drive, first stop Avast from running. Then, shut down the machine. Next, there should be a Recovery Partition on your hard drive. To boot your machine to that partition, hold down the Command and R keys at startup. It will take some time to get to the main menu "screen" for the software on that partition, but eventually, it will come up. Here is a link that talks about this partition:
https://support.apple.com/en-us/HT201314
What you need to do is select Disk Utility from the menu, and have it do the following:
1. At the
Volume level, top) level, have it Verify and Repair the disk.
2. At the
Partition level, ie, the next "level" down, have it 1) Verify and Repair Permissions, and 2) Verify and Repair the disk.
Hopefully, that will give you enough information and/or help with your issues.
Additionally, you should get rid of any unnecessary/no longer needed "stuff" on the drive.
The other thing that could help is to defragment/optimize the drive. The Mac OS "supposedly" does that, but I much prefer to use other software for that purpose. The disk repair/maintenance program Tech Tool Pro is excellent, and has such software. I have been using it for many, many years, and it is one of the products I depend on for keeping my system (on both of my machines) as "mean and clean" as possible. Disk Warrior is another excellent choice. I am not aware of any "decent" freeware software for defragmenting/optimizing drives.
Additionally, you might consider downloading the excellent freeware program Onyx. You can get it from here:
http://www.macupdate.com/app/mac/11582/onyx
(Note that is the version for Yosemite). This venerable gem has been around for a number of years, has excellent reviews, and does a good job with its numerous tasks.
Finally, you might want to consider investing in a 256 gig SSD to install inside you machine. That will make a world of difference regarding speed.
