Firefox and Ubuntu

First of all, my system (at the moment) is as follows:

Ubuntu 14.04 (trusty) with kernel 3.13.0-49-generic (#83-Ubuntu SMP )Gigabyte 956P-DS3 motherboard
8 GBytes of RAM
Intel Core2Duo CPU  6300  @ 1.86GHz
Graphics Processor: GeForce GT 610 with 48 CUDA Cores and 2 GBytes Memory
HD WD500GB SATA
Onboard 10/100/1000baseT 88E8056 PCI-E Gigabit Ethernet Controller vendor: Marvell Technology Group Ltd.
Onboard Intel HD Audio Controller 82801H (ICH8 Family)

Well this last week after, I had five situations where my system freezed (after many years using stable Linux versions).

I had to REISUB to gain controle back and kill Firefox.

I started to check update, upgrades, and finally started to disable Firefox plugins.  I use very few, which I list bellow:

Adblock Plus 2.6.9
Flashblock 1.5.8
Language Pack Install Helper 2.5
Ubuntu Firefox Modification 3.0
Unity Websites integration 2014.08.06.beta
Webmail Ad Blocker 4.28
X-notifier 3.5.6 (manual update mode - not set "Enable")

My X-notifier has many gmail, yahoo, hotmail, ig mail boxes to check.

What I have identified:

When I update the X-notifier by clicking "Check now", the CPU usage goes up fast and never falls below 20% after all is checked.
After that I disable the plugin and restart, as instructed, and again enable and restart again and CPU returns to normal.

Please help, I really like X-notifier.