Can you provide some more info on: number of tabs usually loaded while Browser is up, only ff-dev or ff-dev + Chrome + Opera @same time working with 10-tabs each maybe, do you use hibernation as a power-option etc. lot's of stuff comes to my mind but you need to provide much concise info so we can help you out, after all you are a developer, so I bet you are lazy as myself but sometimes you need to troubleshoot your machine in order to fix it. Go get some more information and post it here.
Ubuntu 16.02 or Ubuntu 16.04.2? What environment are you using, log startup programs and services with blame etc, do some troubleshooting and post us logs and more concise info on your system.
On top of all your Swap partition is too small for your Ram proportions I think. With that SSD you can get much better performance.
I also do the dev-stuff on my laptop but it never goes above 5% processor cores or 2-3GB Ram of 8gb, one module burned recently but works fine with 8gb as well and without SSD in 2nd slot, only HDD on 7200 RPM inside machine and after RAM and SSD burned after electric shock I made SWAP partition for the first time(did not have it at all with 16gb Ram because it never goes above 5gb in my case). I guess you should check out all scenarios & variables and help us help you to solve the problem, myb provide what services/packages you are running often or on system start. I disabled apache2, mongod and MySql servers from starting automaticaly so I pop them On when needed, also NodeJS and much more stuff that can rape that i5 in your rig.
What IDEs are you on, what kind of dev-stuff you do if that's not a secret? Describe standard scenario/situation like you said "this happens after 5min. system start time" what you usually do first 5mins. or just provide startup programs/services with
sudo systemd-analyse blame
Lot's of stuff can do the mess-job there, just need some more information from you to troubleshoot it faster my friend.
Here is my situation with all 4 servers running for 10-15-30min. now and using phpmyadmin for MySql actively also Robomongo - MongoDB Gui with Apache Server and Postgre-9.5, Browsing in Chromium 9-10 tabs and Firefox on, so with almost 300 processes and 10+ panels in top of it and barely 3GB Ram used(that's my high-usage btw.), processor barely above 2% usage as you see.
Here is a screenshot of my current system state, send me feedback if you like or check it out and compare to your usual usage level
Will check back topic later and try to help you troubleshoot this when you provide more info, I am really interested what bothers you that much.
Well that depends on the cloning tools and what type of cloning you did. But let's say you saved your data you don't wanna lose, time for new Partition Table on your SSD, give that machine some breathe bro :) it's choking for no reason. And yes Firefox-dev is a bad optimized browser resources-wise and it just snatches enormous amount of RAM pretty fast, I did use it on my old rig and after a while realized I don't really gain that much out of it, comparing to lighter Chromium + FFox(normal edition) just covers every aspect of web-development testing and with few plugins you even gain much more with less resources eaten by dev-edition. You need to fing a perfect combination of Linux Distro + Desktop Environment for your needs and of course respect the rig requirements and according to that just make your perfect IDE with no headaches like this and morning smile while drinking 1st coffee and being satisfied you can work freely and not be scared of data-loss or hardware failure+data-loss(god forbid) :)
Let me know if you need any help on building your own rig for your own needs and development variation.