High/Lo do all airspace
High does above the countries upper/lower airspace division, if any. This can be anything like 18000, FL195, FL245, FL285 etc. etc
Lo does below the countries upper/lower airspace division, if any. This can be anything like 18000, FL195, FL245, FL285 etc. etc
Lower/Upper ATC may have different divisions that do not conform to these definitions.
Lower/Upper Airways may have different divisions that do not conform to these definitions.
FIR generally up to the limit of the lower airspace. If any. Otherwise entire airspace, sometimes up to UNL.
UIR generally above the upper limit of the FIR.
No FIR/UIR in the US. What the real domestic purposes of these are remains unclear these days as other airspace and ATC sectors most likely do not coincide with domestic FIR/UIR borders.
All clear?