After upgrading from Ubuntu 16.04 to Ubuntu 18.04, may things are better -- but my computer randomly freezes occasionally, requiring a reboot.

Not good.

Help appreciated.

-- lubkin

Not enough information.

Not good.

Troubleshooting required.

-- o.

My problem was that the computer froze randomly, requiring a reboot. (Hp laptop, Ubuntu 18.04.)
There was another problem as well -- while scrolling thru thunderbird email messages, some large annoying emojis appeared.
I saw elsewhere in this forum that others had this problem -- they referred to the emoji files stored in /usr/share/fonts/truetype/noto. I went to this directory, and moved the two Emoji files stored there to an innocuous location in my Dropbox. The annoying large Emojis in Thunderbird disappeared. I still had the problem of the computer randomly freezing.
I rebooted.
Both problems haven't happened since; computer doesn't randomly freeze; and ugly large Emojis don't appear in thunderbird.
I don't know why.
Also, I had supplied some misinformation: I did not upgrade from 16.04 to 18.04 on this computer; that happened on two other computers that I own -- which had no problems.
On the laptop which had the random freeze / unwanted large Emoji problems -- I did a fresh install of Ubuntu 18.04, using an installation DVD that I burned on another computer. My install is not dual boot (it's dual boot on my other two Ubuntu 18.04 computers.)
Also, the other two Ubuntu 18.04 computers -- which had no problems -- have the same directory, /usr/share/fonts/truetype/noto on them, filled with the same large Emoji files -- but they don't cause random freezes or annoying large emojis when scrolling thru thunderbird.

Information from someone more knowledgeable than myself would be appreciated -- and might be of use to other Ubuntu 18.04 users, who installed Ubuntu 18.04 as a single boot system.

Problem solved!
I saw a related problem in the Ubuntu One Forum:

Following the recommendations in that post, I installed fonts-symbola:
sudo apt install fonts-symbola

I then copied all the files back to /usr/share/fonts/truetype/noto. AND, now, there are no longer huge Emojis showing when I scroll thru thunderbird -- AND I haven't had any freezing of my computer.

-- Saul