Get the Exclusion List

# yum update

yum list all updates, choose which you don't want update.

Update without some packages

In command line

# yum --exclude=firefox\* --exclude=thunderbird\* update

or:

# yum -x 'firefox*' -x 'thunderbird*' update

Disable them permanently

Add the following line into /etc/yum.conf:

exclude=firefox* thunderbird*

Ref:

1. Disable Certain Package Updates using YUM in RHEL / CentOS / Fedora

2. RHEL/CentOS: yum Update All Packages Except the Linux Kernel

3. Force yum update Command To Exclude Certain Packages