firefox will not start after latest update.

firefox: /usr/lib/libstdc++.so.6: version `GLIBCXX_3.4.21' not found (required by firefox)

where to get this glibcxx, i can't find in repos?

Please post the link Octopi > Tools > SysInfo produces.

Error: Got Net::HTTPForbidden from gist: {"message":"API rate limit exceeded for 206.41.240.18. (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)","documentation_url":"https://developer.github.com/v3/#rate-limiting"}

is this a huge pain to get an authenticated request?

this one will have to just sit as it looks too involved for me to get into, looks to be beyond me from what I can tell going to developer.github.com.

Then start with posting parts of what that tool gives:

pacman -Qm
kate /etc/pacman.conf

And for this case (hit enter to get the full output):

strings /usr/lib/libstdc++.so.6 |grep GLIBCXX |less
$ pacman -Qm
attica 0.4.2-2
breeze4 5.5.3-1
calligra-l10n-en_GB 2.9.9-1
cloog 0.18.1-5
imagewriter 1.10.11-1
isl 0.13-3
kdelibs 4.14.16-1
kdepimlibs 15.12.1-1
kget 15.07.71-2
libdbusmenu-qt 0.9.2-2
libqzeitgeist 0.8.0-4
phonon 4.8.3-1
phonon-backend-gstreamer 4.8.2-1
polkit-qt 0.112.0-1
pyqt-python2 4.11.4-1
qca 2.1.1-1
qt 4.8.7-2
qt5-quick1 5.5.1-1
qtcurve4 1.8.18.4-1
qtwebkit 2.3.4-2
strigi 0.7.8-10

output kate /etc/pacman.conf

#
# /etc/pacman.conf
#
# See the pacman.conf(5) manpage for option and repository directives

#
# GENERAL OPTIONS
#
[options]
# The following paths are commented out with their default values listed.
# If you wish to use different paths, uncomment and update the paths.
#RootDir     = /
#DBPath      = /var/lib/pacman/
#CacheDir    = /var/cache/pacman/pkg/
#LogFile     = /var/log/pacman.log
HoldPkg     = pacman glibc
#XferCommand = /usr/bin/wget --passive-ftp -c -O %o %u
#XferCommand = /usr/bin/curl -C - %u > %o
#CleanMethod = KeepInstalled
#UseDelta    = 0.7
Architecture = auto

# Pacman won't upgrade packages listed in IgnorePkg and members of IgnoreGroup
#IgnorePkg   =
#IgnoreGroup =

#NoUpgrade   =
#NoExtract   =

# Misc options (all disabled by default)
#UseSyslog
#ShowSize
Color
#TotalDownload
CheckSpace
VerbosePkgLists
ILoveCandy

# By default, pacman accepts packages signed by keys that its local keyring
# trusts (see pacman-key and its man page), as well as unsigned packages.
SigLevel = Never
LocalFileSigLevel = Never
#RemoteFileSigLevel = Required

#
# REPOSITORIES
#   - can be defined here or included from another file
#   - pacman will search repositories in the order defined here
#   - local/custom mirrors can be added here or in separate files
#   - repositories listed first will take precedence when packages
#     have identical names, regardless of version number
#   - URLs will have $repo replaced by the name of the current repo
#   - URLs will have $arch replaced by the name of the architecture
#
# Repository entries are of the format:
#       [repo-name]
#       Server = ServerName
#       Include = IncludePath
#
# The header [repo-name] is crucial - it must be present and
# uncommented to enable the repo.
#

[core]
Include = /etc/pacman.d/mirrorlist

[main]
Include = /etc/pacman.d/mirrorlist

[apps]
Include = /etc/pacman.d/mirrorlist

# An example of a custom package repository.  See the pacman manpage for
# tips on creating your own repositories.
#[custom]
#Server = file:///home/custompkgs

for strings /usr/lib/libstdc++.so.6 |grep GLIBCXX |less

GLIBCXX_3.4
GLIBCXX_3.4.1
GLIBCXX_3.4.2
GLIBCXX_3.4.3
GLIBCXX_3.4.4
GLIBCXX_3.4.5
GLIBCXX_3.4.6
GLIBCXX_3.4.7
GLIBCXX_3.4.8
GLIBCXX_3.4.9
GLIBCXX_3.4.10
GLIBCXX_3.4.11
GLIBCXX_3.4.12
GLIBCXX_3.4.13
GLIBCXX_3.4.14
GLIBCXX_3.4.15
GLIBCXX_3.4.16
GLIBCXX_3.4.17
GLIBCXX_3.4.18
GLIBCXX_3.4.19
GLIBCXX_3.4.20
GLIBCXX_DEBUG_MESSAGE_LENGTH
_ZNKSt14basic_ifstreamIcSt11char_traitsIcEE7is_openEv@GLIBCXX_3.4
_ZNSt13basic_istreamIwSt11char_traitsIwEE6ignoreEv@@GLIBCXX_3.4.5
_ZNKSbIwSt11char_traitsIwESaIwEE11_M_disjunctEPKw@GLIBCXX_3.4
_ZNKSt14basic_ifstreamIwSt11char_traitsIwEE7is_openEv@@GLIBCXX_3.4.5
GLIBCXX_3.4.5
GLIBCXX_3.4.9
_ZSt10adopt_lock@@GLIBCXX_3.4.11
GLIBCXX_3.4.10
GLIBCXX_3.4.16
GLIBCXX_3.4.1
_ZNSt19istreambuf_iteratorIcSt11char_traitsIcEEppEv@GLIBCXX_3.4
_ZNSs7_M_copyEPcPKcm@GLIBCXX_3.4
_ZNSt19istreambuf_iteratorIcSt11char_traitsIcEEppEv@@GLIBCXX_3.4.5

sorry i don't know how to put text in boxes, are there controls here anywhere?

How you perform system update?

See the banner on this forum for the link how to use markdown (code tags are 3 back ticks, done for you this time)
You're system is not up to date at all, not even clear what you installed, which ISO did you use?
And make sure to run:

sudo pacman -Syu

partial updates are never supported, read the pacman guide:
https://kaosx.us/docs/pacman/

I had the issue over the last weekend develop. It was used for several months and as of last Saturday was completely up to date. FF suddenly quit working saturday and gave this glibxx error message, but worked for several months without issue.

As for the not being up to date now, I backed up my files and installed an ssd so started with a fresh system here today. right now my internet connection will not support doing 736 updates. the error message is still the same glibxx thing. I will take home to a better connection, update, and try again. usually update using the octopi in the tray...sync database and then system upgrade.

That is a partially update.....
You can not run this system without fully updating. It shows you are running on a complete outdated glibc (older than 6 months), which is the very base of the system.
Your system will break irreparably if you continue this way, ALWAYS fully update (sudo pacman -Syu).