I followed this and reduced my profile folder by 40 Mb.
sonicwind is offline Just Give Me the Beans!
Ubuntu 16.04 Xenial Xerus
Re: mozilla is gowing very large.
I just went through this same issue and got help from the folks at the mozillazine forums. They were very helpful. Here is what I learned and did to correct this.
1) BACK UP your profile first.
2) You can delete the "webappstore.sqlite" file in your profile folder. It will re-create.
3) You can delete the "datareporting", "storage", "minidumps" and "saved telemetry pings" folders. They will re-create again.
4) Go to about:support in Firefox (or you can get there by --> Help --> Troubleshooting). Scroll down the page near the bottom and find the "Places Database" with a "Verify Integrity" button. Click the button, it will vacuum your places.sqlite file and do an integrity check right there on the page. It will show you how much it reduced that file by afterwards.
5) Restart Firefox and make sure everything works with the slimmed down profile. Then make a new backup of your slimmed-down profile.
Doing all of this, I reduced my profile folder from almost 400mb and 17,000 files to about 110mb and 234 files. I plan to take these steps at least once or twice a year now. I hope this helps.