Hangs up, and I have yet to pinpoint the moment, except that Firefox is always running when it does, but then FF is always running!
Could it be the video drivers?
Brand new Crucial_CT275MX3 SSD.
What output would anyone like to see?

Thank you all...

Here is my system info:

System: Host: XXXXXX-ThinkPad-T61 Kernel: 4.10.0-38-generic x86_64 (64 bit gcc: 5.4.0)
Desktop: Cinnamon 3.4.6 (Gtk 3.18.9-1ubuntu3.3) dm: lightdm Distro: Linux Mint 18.2 Sonya
Machine: System: LENOVO (portable) product: 64574UU v: ThinkPad T61 Chassis: type: 10
Mobo: LENOVO model: 64574UU Bios: LENOVO v: 7LETB9WW (2.19 ) date: 06/06/2008
CPU: Dual core Intel Core2 Duo T7300 (-MCP-) cache: 4096 KB
flags: (lm nx sse sse2 sse3 ssse3 vmx) bmips: 7980
clock speeds: min/max: 800/2001 MHz 1: 1600 MHz 2: 1200 MHz
Graphics: Card: NVIDIA G86M [Quadro NVS 140M] bus-ID: 01:00.0 chip-ID: 10de:0429
Display Server: X.Org 1.18.4 drivers: nouveau (unloaded: fbdev,vesa)
Resolution: 1680x1050@59.98hz
GLX Renderer: Gallium 0.4 on NV86 GLX Version: 3.0 Mesa 17.0.7 Direct Rendering: Yes
Audio: Card Intel 82801H (ICH8 Family) HD Audio Controller
driver: snd_hda_intel bus-ID: 00:1b.0 chip-ID: 8086:284b
Sound: Advanced Linux Sound Architecture v: k4.10.0-38-generic
Network: Card-1: Intel 82566MM Gigabit Network Connection
driver: e1000e v: 3.2.6-k port: 1840 bus-ID: 00:19.0 chip-ID: 8086:1049
IF: enp0s25 state: down mac: <filter>
Card-2: Intel PRO/Wireless 4965 AG or AGN [Kedron] Network Connection
driver: iwl4965 v: in-tree: bus-ID: 03:00.0 chip-ID: 8086:4230
IF: wls3 state: up mac: <filter>
Drives: HDD Total Size: 775.2GB (26.5% used)
ID-1: /dev/sda model: Crucial_CT275MX3 size: 275.1GB serial: 164114385E23
ID-2: USB /dev/sdb model: Simple_Drive size: 500.1GB serial: WD-WCAV92135067
ID-3: /dev/mmcblk0 model: N/A size: 2.0GB serial: 0x7026c60b
Partition: ID-1: / size: 250G used: 19G (8%) fs: ext4 dev: /dev/dm-0
ID-2: /boot size: 472M used: 176M (40%) fs: ext2 dev: /dev/sda1
ID-3: swap-1 size: 2.11GB used: 0.04GB (2%) fs: swap dev: /dev/dm-1
RAID: System: supported: N/A
No RAID devices: /proc/mdstat, md_mod kernel module present
Unused Devices: none
Sensors: System Temperatures: cpu: 56.0C mobo: 54.0C gpu: 66.0
Fan Speeds (in rpm): cpu: N/A fan-1: 3091
Repos: Active apt sources in file: /etc/apt/sources.list.d/flexiondotorg-youtube-dl-gui-xenial.list
deb http: //ppa.launchpad.net/flexiondotorg/youtube-dl-gui/ubuntu xenial main
deb-src http: //ppa.launchpad.net/flexiondotorg/youtube-dl-gui/ubuntu xenial main
Active apt sources in file: /etc/apt/sources.list.d/official-package-repositories.list
deb http: //packages.linuxmint.com sonya main upstream import backport #id:linuxmint_main
deb http: //archive.ubuntu.com/ubuntu xenial main restricted universe multiverse
deb http: //archive.ubuntu.com/ubuntu xenial-updates main restricted universe multiverse
deb http: //archive.ubuntu.com/ubuntu xenial-backports main restricted universe multiverse
deb http: //security.ubuntu.com/ubuntu/ xenial-security main restricted universe multiverse
deb http: //archive.canonical.com/ubuntu/ xenial partner
Active apt sources in file: /etc/apt/sources.list.d/skype-stable.list
deb [arch=amd64] https: //repo.skype.com/deb stable main
Info: Processes: 202 Uptime: 32 min Memory: 1371.6/1965.1MB
Init: systemd v: 229 runlevel: 5 default: 2 Gcc sys: 5.4.0
Client: Unknown python2.7 client inxi: 2.2.35

Did it ever work?

The new ssd worries me most. Some are really too fast for some systems that were never designed to use them.

Can't hurt to run memtest for a day.

If only firefox then maybe I'd look there for some ideas. Also maybe ram use during. Performance monitor or top.

/var/logs may hold clues too.

It could be a graphics issue yes.

Your GPU is Nvidia:-

Try running the driver manager and install the Nvidia drivers. Once installed reboot the system.

I'm not an expert with Nvidia.

Here's an example:
https://johners.tech/2017/01/11/inst...linux-mint-18/

Look in dmesg too.

This dmegs appears over and over;

21017.799555] sd 2:0:0:0: [sdb] tag#0 FAILED Result: hostbyte=DID_ERROR driverbyte=DRIVER_SENSE
[21017.799560] sd 2:0:0:0: [sdb] tag#0 Sense Key : Hardware Error [current] [descriptor]
[21017.799563] sd 2:0:0:0: [sdb] tag#0 Add. Sense: No additional sense information
[21017.799567] sd 2:0:0:0: [sdb] tag#0 CDB: ATA command pass through(12)/Blank a1 06 20 da 00 00 4f c

sdb is a usb and that error is common. Doubt it has anything to do with your issue. Does this usb need to be attached for testing your problem?

I've installed the Nvidia driver, and Firefox was updated, so I'll see what happens.
Thanks all!

Don't forget to restart your pc after installing the Nvidia driver:-